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Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

Microsoft  Corporation 


http://www.archive.org/details/artofprojexperiOOdolbrich 


The  Art  of  Projecting. 


PHYSICS,  ^^f 

Chemistry,  and  Natural  History 


WITH     THK 


PORTE  LUMIERE  AND  MAGIC  LANTERN. 

BY 
PROF.  A.  E.  ^OLBEAR,  M.E.,  Ph.D. 

(tufts  college.). 

Neto  Edition,  laeijiseU,  toitfj  Munitions. 

ILLUSTRATED. 


BOSTON  1892 
LEE  AND  SHEPARD  PUBLISHERS 

10  MILK  STREET  NEXT  "  THE  OLD  SOUTH  MEETING  HOUSE  " 


K 


Copyright,  1877, 
By  Lee  and  Shei-ard. 

Copyright,  1887, 
By  Lee  and  Shepard. 


All  rights  reserved. 
Art  of  Pkoiecting. 


S      J       PARKHILL    &    CO.,     PKINTERS 
BOSTON 


PREFACE   TO   THE   SECOND   EDITION. 


Since  the  first  publication  of  this  book  the  author  has  received 
so  many  commendatory  letters  from  many  parts  of  the  world  that 
he  is  fully  persuaded  that  the  book  met  a  real  want ;  and  a  sun- 
beam is  now  made  useful  in  school  work  and  in  the  study  of 
phenomena  in  many  places  where  no  substitute  is  practicable.  In 
preparing  a  new  edition,  some  things  have  been  added  which  it  is 
hoped  will  make  the  book  still  more  useful  to  such  as  consult  it. 
Two  things  may  be  specially  mentioned  here :  the  electric  lamps 
and  lights  for  projection  purposes,  and  the  production  and  phe- 
nomena of  vortex  rings.  Of  the  former  there  is  pointed  out 
what  is  at  present  practicable,  and  of  the  latter  it  may  be  said  that 
the  vortex-ring  theory  of  the  constitution  of  matter  has  so  much 
philosophical  as  well  as  scientific  importance,  and  the  phenomena 
presented  by  vortex  rings  are  so  curious  and  unexpected,  that  the 
author  has  felt  warranted  in  presenting  what  he  believes  to  be  the 
most  complete  series  at  present  known,  especially  as  he  believes 
himself  to  be  the  discoverer  of  a  considerable  number  of  them. 

Several  small  treatises  on  the  management  and  use  of  lan- 
terns have  been  published  lately,  and  may  be  had  on  application  to 
almost  any  of  the  larger  dealers  in  physical  instruments  and  lan- 
tern transparencies.  It  was  not  thought  advisable  to  add  anything 
on  that  subject.  An  excellent  manual  for  experimental  work  with 
the  lantern  has  been  published  by  Lewis  Wright,  which  every  one 
interested  in  such  matters  should  have,  especially  as  many  of  the 
experiments  described  in  it  can  be  done  as  well  or  better  by  the 
use  of  a  beam  of  sunlight,  —  the  use  of  a  beam  of  sunlight  for 
projection  being  the  peculiar  province  of  this  book. 

A.    E.   DOLBEAR 
College  Hill,  Mass., 
Sept.  Qth,  1887. 


42'lSii 


INDEX. 


Absorption  spectra     ....  114 

Acoustic  curves 61 

Air  thermometer    .....  144 

Animalcule  cage 33 

Arc  light,  To  project  ....  161 

Biaxial  crystals 132 

Bubbles 107 

Bubbles,  cohesion  of  .    .    .    .168 

Calorescence •  149 

Camphor  on  water      ....  47 

Camera  obscura ......  80 

Candle  power 13 

"       flame,  To  project.    .132,100 

Capillarity 49 

Caustics  by  reflection      ...  92 

"        "    refraction      ...  104 

Chameleon  top 143 

Chemical  tank 34 

"         reactions      ....  157 
Chladni's  experiment      ...  62 
Chromatic  aberration      ...  104 
Chromatic  aberration,  lessen- 
ing of   169 

Chromatrope  .     ......    .  142 

Cloud  formation 145 

Cohesion 45 

Cohesion  figures 47 

College  lantern 41 

Colors  of  thin  films     ....  107 
Concave    mirror,    To   project 

with 63,  91 

Convection  in  water  ....  156 

"  air 98 

Condenser:  its  use      ....  26 

Convex  mirrors 93 

Crova's  apparatus 77 

Crystalline  substances  for  po- 
larized light 133 

Darkened  room 5 

Diagrams  on  mica 129 

Diamagnetism 151 

Diffraction 137 

Disks  for  study  of  colors  .      110,  143 

Dispersion 105 

Distortion 93 

Divisibility  of  matter  ....  44 

Double  refraction 126 

Double  salts.  Prepared   .    .    .  134 

Drummond  light 11 


Eidotrope 42 

Electric  light 9 

"    To  project  ...  163 

Electric  spark,  To  project  .    .  166 

Elements,  Spectra  of  ....  165 

Engravings,  To  transfer      .    .  32 

Etcliing  upon  glass     ....  31 

Films,  Vibration  of    ....  169 

Floating  magnets 167 

Fluorescence 119 

Focal  length  of  lenses     ...  21 

Focusing 25 

Fountain,  Illuminated    ...  96 

Fraunhofer's  lines Ill 

Galvanometer 147 

Gases  for  lime  light    ....  11 

Ghost      .    • 84 

Glue,  Marine 35 

Gramme  machine 9 

Gravitation 50 

Heat  ........      144,  155 

Heliostat 1 

Ice  flowers 52 

Illumination,  Intensity  of  .    •  81 
Images  formed  by  lenses    .    .  100 
Incandescent  electric  lamp  .    .  163 
"       lamp  fil- 
aments   164 

Incandescent    electric    lamp, 

Currents  for 164 

Interference 71 

'•         spectra     ....  118 

Interlacing  lines 70 

Kaleidoscope 88 

Kaleidophone 57 

Lamps,  Electric  arc    .    .      159,  160 

"        Incandescent.    ...  163 

Landscape  projection ....  170 

Lanterns 14,  18,  19 

Oxyhvdrogen,  12, 16, 18,  19 

Electric 161 

Lenses •    ..    .  19 

"      Magnifying  power    .    .  .33 

"      Mountings  for  ....  23 

Light 80 

'•     Intensity  of 13 


vi 


INDEX. 


Light,  Magnesium 10 

"       Lime 11 

"       Composition  of,  109,  117,  136 

«•       Polarized 127 

Lissajous'  experiments  ...  09 

Mach's  experiment      ....  64 

Magnetism 150 

Magnetic  phantom 150 

Manometric  flames      ....  62 

Marine  ghie 35 

MegascoiJe 38 

Melde's  experiment    .     .     .    •  58 

Microscope,  8olar 100 

"            attachment ...  49 

Minute  substances 1.33 

Mirage 95 

Monochromatic  light .    .      108,  V£l 

Newton's  disk 143 

"          rings 109 

Objective 25 

Objects  for  projection      ...  27 

Organ  pipe 65 

Opeidoscope 59 

Outline  drawings 29 

Overtones 71 

Persistence  of  vision  ....  139 

Pepper's  ghost 84 

Plateau's  (experiment)  ...  56 

Polarization  of  light  ....  127 

Porosity 45 

Porte  Lumiere,  To  make     .    .  2 
"             "         its  use    ...  24 
Porte  Lumieres,  various  pat- 
terns       2,  3 

Projection  with  single  lens      .  24 
'*               "    condenser  .     .  27 
"           of  large  apparatus,  35 
"           Apparatus  for  ver- 
tical    40 

Pyrometer 145 

Rainbow 100 

Reactions,  Chemical  ....  157 

Reflections 82 


Reflections,  Multiple  ....  83 

Refraction 97 

Resultants 72 

Salicine  crystals 134 

Screens  6 

Sciopticons 18 

Silver  crystals 53 

Singing  flames 64 

Sinuous  lines 69 

Sizes  of  objects  and  of  images, 

To  compute 171 

Soap  bubbles,  Persistent     .    .  108 

•'           "        Tension  of    .     .  107 

Sodium  line  in  solar  spectrum,  170 

Solar  microscope 100 

"      spectrum Ill 

Spectacle  glasses.  To  test    .    .  132 

Spheroidal  form 62 

Spectra,  Methods   of  project- 
ing          121,  153 

Spectrum  analysis 119 

"          of  sodium    ....  121 

*'          "       "      reversed  .  122 

Starch 134 

Stroboscope 139 

Sympathetic  vibrations     ...  75 

Thermometer 144 

Total  reflection t)4 

Tuning  forks 57 

Vibrations  of  strings  ....  59 
"             "  forks      ....  57 
Vision,  Persistence  of     .     .    .  139 
Vortex  rings,  To  produce    .     .  172 
"          "        their     phenom- 
ena    174 

Water,  Decomposed    ....  153 

"       Maximum  density   .    .  146 

Refraction  of  ...     .  97 

"       Total  reflection  in    .     .  94 

Waves  in  water 61 

Whirling-table  attachment      .  77 

Zoetrope 140 


THE  ART  OF  PROJECTING. 


A  MAGNIFIED  image  of  a  picture,  or  of  any  phenom 
eiion,  when  thrown  upon  a  screen  by  means  of  sunlight, 
and  lenses,  or  with  a  magic  lantern,  is  called  a  projec- 
tion. 

When  sunlight  is  to  be  used  for  this  purpose,  it  is 
necessary  to  have  some  fixture  to  give  the  proper  direc- 
tion to  the  beam.  The  helinstat  and  the  porte  lutniere 
are  the  devices  in  common  use.  The  latter  was  the 
earliest  form,  and  was  invented  by  Gravesand,  a  Dutch 
professor  of  natural  philosophy,  in  the  early  part  of  the 
last  century.  It  was  afterwards  reinvented  by  Captain 
Drummond,  an  Englishman,  who  called  it  the  heliostat. 
The  latter  term  is  now  only  applied  to  an  automatic 
arrangement,  by  which  a  mirror  is  moved  by  clock- 
work in  such  a  way  that  a  beam  of  sunlight  reflected 
from  it  may  be  kept  in  one  direction  all  day,  if  it  be 
needed  so  long.  Silberman  and  Foucault  have  each 
devised  very  satisfactory  instruments,  but  they  are  too 
costly  to  be  owned  by  any  but  the  wealthy ;  the  catalogue 
price  of  the  cheapest  of  these  being  five  hundred 
francs.  C.  Gerhardt,  of  Bonn,  however,  makes  a  small 
one,  carrying  a  good  mirror  three  inches  in  diameter, 
for  twenty  dollars. 

THE    PORTE    LUMIERE HOW  MADE. 

The  poi'te  lumiere  is  made  o^  various  patterns  by 
different  makers,  but  the  differences  consist  chiefly  in 


2  THE  ART  OF  PROJECTING. 

the  devices  for  giving  proper  movements  to  the  mirror. 
Their  cost  is  from  ten  to  twenty  dollars  according  to 
their  size,  workmanship,  and  attachments.  On  the 
opposite  page  are  engravings  of  several  such  as  are  in 
the  market.  It  is  recommended  that  one  be  purchased 
at  the  outset,  if  it  can  be  afforded,  but  as  many  who 
would  be  glad  to  work  with  one  may  not  be  able  to 
purchase  it,  directions  will  be  given  for  making  one  that 
will  enable  any  person  who  is  familiar  with  the  use  of 
carpenters'  tools  to  make  one  at  a  trifling  cost  that  will 
answer  for  many  purposes. 

The  room  in  which  the  porte  lumiere  is  to  be  used 
must,  of  course,  be  one  into  which  the  sun  can  shine. 
A  room  having  windows  only  upon  the  North  side, 
evidently  cannot  be  used  at  all  for  such  a  purpose  ;  one 
having  windows  only  upon  the  East  or  upon  the  West 
side  could  be  used  only  in  forenoon  or  afternoon  ;  while 
one  with  windows  looking  to  the  South  can  be  used 
nearly  all  day.  Choose  then  that  window  where  the 
sun  is  available  the  longest,  and  opposite  to  which  can 
be  stretched  the  screen  to  receive  the  projections  upon. 
Next,  take  a  well-seasoned  piece  of  pine  board  a  foot 
or  more  in  width,  and  an  inch  thick  when  dressed  ; 
cut  it  to  the  length  of  the  width  of  the  window  sash, 
so  that  it  may  fit  into  the  window  frame,  and  the  sash 
be  brought  down  upon  it ;  this  will  keep  it  tightly  in 
place.  With  the  compasses,  scratch  two  concentric 
circles  in  the  middle  of  the  board,  one  with  a  radius  of 
four  inches,  the  other  with  a  radius  of  four  inches  and 
a  half.  Saw  out  the  inner  circle  completely,  and  cut 
the  other  but  one  half  through  the  board,  and  then  cut 
away,  making  a  square  rabbet,  as  shown  at  b  b.  Next, 
take  a  round  piece  of  inch  board  of  the  same  diametei 


Hawkkidge's.      ^ 


Queen  &  Co.'s, 


port:6  lumieres. 


'"i'!ll!!|||||||!i'liiir'^""    '      ""'"'I*"*" 

Adams's. 


Strong's. 
PORT]il  LUMIERES. 


PROJECTIONS  FOR    THE  SCHOOL-ROOM.         3 

as  the  outer  circle  (namely,  nine  inches),  cut  a  rabbet 
upon  one  side  of  it  so  that  it  will  nicely  fit  into  the  hole 
of  the  larger  board,  as  indicated  2Xc  c. 

Make  the  worked  edges,  and  touching  surfaces,  quite 
smooth ;  but  the  outer  edge  should  be  made  a  trifle 
smaller  than  the  hole,  in  order  to  allow  the  disk  to 
turn  freely  round  in  it ;  then  the  hole  may  be  cut  in  the 
disk  to  receive  the  lens,  four  or  five  inches  in  diameter, 
whichever  it  may  chance  to  be. 

Procure  a  nice  piece  of  thin  looking-glass,  twelve  or 
fifEeen  inches  long  and  five  inches  wide.  Fasten  it  to 
a  back  of  wood  made  a  little  larger  than  itself,  with 
broad-headed  tacks,  or  bits  of  wire  driven  in  and  the 
top  bent  at  right  angles.  This  back  will  need  to  be 
an  inch  thick  at  the  bottom,  but  may  taper  like  a 
shingle  to  the  top,  where  it  need  not  be  half  an  inch 
thick ;  m  is  the  mirror  and  //  is  the  back  in  the  figure 
adjoining. 

A  common  desk  hinge  h  may  be  used  to  attach  this 
mirror- mounting  to  the  part  c  in  the  figure  below.  It 
must  be  so  fastened  that  the  mirror  may  swing  through 
ninety  degrees  from  a  horizontal  plane.  The  accom- 
panying figure  will  be  sufficiently  definite  to  enable 
any  one  to  make 
the  whole  instru- 
ment. When  the 
mirror  is  securely 
fastened  to  the 
part  c,  the  whole 
can  be  inserted  in 
the  board  /^  ^  and 
buttoned  in,  as  is 
shown  at  b  and  b ; 
these  buttons  must 


4  THE  ART  OF  PROJECTING, 

not  bind  upon  the  part  c,  as  this  must  have  an  easy 
rotation  in  its  place,  though  they  need  to  be  tight  in 
the  board  b ;  three  of  them  will  be  enough.  Again, 
a  string  must  be  attached  to  the  end  of  m,  passed 
through  a  small  hole  in  <r,  and  tied  to  a  tight-fitting 
thumb  peg  at  d.  As  the  peg  is  turned  the  mirror  will 
be  raised  or  lowered.  A  short  lever  v  must  be  made 
fast  to  some  part  of  c  with  which  to  turn  the  whole  fix- 
ture around  as  the  sun  moves.  The  ray  of  light  /  can 
then  be  always  kept  where  it  is  wanted. 

If  the  window-sill  be  no  more  than  two  or  three  feet 
from  the  floor,  it  will  be  better  to  have  this  fixture 
either  put  into  a  window  shutter,  or  to  remove  a  pane 
of  glass  at  the  proper  place  and  fasten  the  board  b  b 
into  it.  In  this  case  it  will  be  necessary  to  have  a  cap 
to  place  over  the  hole  when  it  is  not  in  use. 

The  lenses  will  need  to  be  purchased;  and  for  a 
beginning  I  recommend  a  cosmorama  lens  five  Or  six 
inches  in  diameter  and  with  a  focal  length  of  eighteen 
or  twenty  inches  ;  a  plano-convex  lens  of  two  and  a 
half  or  three  inches  diameter  and  eight  or  nine  inch 
focus ;  also  a  pocket  botanical  glass  with  focus  of  one 
or  two  inches.  These  three  lenses  should  cost  no 
more  than  six  dollars,  if  the  two  former  are  unmounted. 
If  one  has  got  a  magic  lantern,  or  a  sciopticon,  the 
lenses  in  that  will  answer  admirably.  Take  one  of  the 
glasses  of  the  compound  condenser  and  fasten  it  into 
the  orifice  of  Xh^porte  lumiere  with  its  convex  side  out ; 
then,  taking  out  the  front  lenses,  hold  them  with  one 
hand  in  the  path  ot  the  divergent  beam  of  light  from 
thQ  porte  lumiere,  and  distant  but  four  or  five  inches 
from  it,  and  with  the  other  hand  hold  some  object 
between  it  and  the  larger  lens ;  by  moving  the  lens  or 
the  object  a  little,  a  sharp  outline  of  it  will  be  observed 


PROJECTIONS  FOR    THE  SCHOOL-ROOM.         5 

upon  the  opposite  wall,  and  then  will  be  seen  what 
further  conveniences  will  be  wanted,  such  as  curtains, 
screen,  table,  mounting  for  lenses,  etc. 

THE  DARKENED   ROOM. 

Exhibitions  with  the  stereopticon  are  almost  always 
given  at  night,  and  there  is  no  trouble  from  exterior 
light ;  but  the  illustrations  and  demonstrations  which 
are  part  of  the  work  of  schools  and  colleges  need  to  be 
given  in  the  daytime,  and  this  necessitates  a  provision 
for  shutting  out  the  light  which  will  interfere  with  the 
experiment. 

The  light  may  be  excluded  from  a  room  by  tight- 
fitting  shutters,  or  with  curtains  It  is  very  difficult  to 
make  shutters  so  tight  that  all  light  is  excluded  by 
them.  It  can  be  done  much  better  and  cheaper  by 
having  some  frames  made  the  size  of  the  window  frames, 
and  covering  them  with  what  is  known  as  enamelled 
cloth,  such  as  is  used  in  upholstery  and  carriage  trim- 
ming. These  should  fit  tight  enough  in  their  places  to 
remain  when  placed. 

The  same  kind  of  cloth  can  be  attached  to  common 
curtain  fixtures,  and  rolled  up  and  down  as  wanted ; 
but  it  will  be  found  that  a  great  deal  of  light  will  pass 
by  the  edge  of  these  curtains.  This  can  be  obviated 
by  tacking  strips  of  the  same  material  a  foot  wide  to 
the  side  of  the  casing,  so  that  the  curtain  will  roll  down 
inside  of  the  strips.  When  sunlight  or  the  lirae-Iight 
is  used,  it  is  not  always  necessary  that  the  room  should 
be  totally  dark ;  and,  indeed,  some  of  the  best  experi- 
menters think  it  a  part  of  their  success  that  their  work 
is  done  in  a  room  that  is  light  enough  for  one  to  see  to 
read  a  newspaper.  Yet  there  are  some  experiments 
which  require  that  extraneous  light  be  shut  out  from 


6  TIJE  ART  OF  PROJECTING. 

the  room :  for  instance,  the  projection  of  the  Fraunho- 
fer  lines  in  the  spectrum  of  the  sun,  and  the  phenomena 
of  diffraction.  For  these,  and  the  like,  the  darker  the 
room  the  better. 

The  curtain  in  the  window  that  holds  the  porie  lumi- 
ere  will  need  to  have  a  hole  cut  in  it  large  enough  to 
allow  the  beam  of  light  to  come  through,  and  to  permit 
the  hand  to  give  proper  motions  to  the  mirror.  A  flap 
should  hang  over  this  when  sunlight  is  not  wanted,  and 
the  electric  light  or  the  lime-light  is  used  instead. 

THE  SCREEN. 

The  white  surface  that  receives  the  projected  picture 
is  called  the  screen,  and  it  may  be  a  white  finished 
wall,  or  white  cloth  properly  mounted.  The  back  of  a 
large  wall-map  makes  a  good  screen  if  the  light  is  used 
in  front  of  it,  and  only  a  small  disk  of  light  is  needed, 
but  the  backs  of  such  maps  are  apt  to  get  discolored, 
and  to  become  so  dark  as  to  be  useless.  They  may 
then  be  made  white  by  painting  them  with  whiting, 
mixed  in  a  thin  solution  of  glue. 

For  a  parlor  exhibition,  a  common  sheet  may  be 
hung  against  the  wall,  or  between  the  folding  doors, 
and  the  lantern  used  on  either  side.  If  the  lantern  is 
placed  back  of  the  screen,  the  latter  should  be  kept 
wet,  as  it  is  made  more  translucent,  and  the  pictures 
will  appear  brighter. 

When  the  porte  lumiere,  the  electric  light,  or  the  oxy- 
hydrogen  lantern  is  used,  a  much  larger  screen  will  be 
necessary.  They  are  sometimes  made  twenty-five  feet 
square  or  more,  but  for  most  purposes  a  screen  fifteen 
feet  square  will  be  large  enough.  Common  bleached 
sheeting,  ten  quarters  wide,  can  be  bought  in  most 
towns.     A  strip  of  this,  ten  yards  long,  cut  into  two 


PROJECTIONS  FOR    THE  SCHOOL-ROOM.  7 

pieces  of  equal  length,  and  having  the  selvedges  sewed 
together,  will  make  such  a  screen  with  but  one  seam. 
That  these  edges  may  come  together,  but  not  lap,  let 
the  sewing  be  done  with  what  is  called  the  carpet  stitch. 
Seme  loops  of  tape  or  small  rings  may  be  sewed  into 
the  corners,  and  it  may  be  hung  upon  nails  driven  into 
the  wall  at  the  proper  places. 

It  is  often  convenient  to  have  the  screen  so  mounted 
as  to  permit  it  to  be  rolled  up  when  not  in  use,  and 
various  devices  have  been  invented  to  effect  this.  Per- 
haps the  neatest  is  to  have  a  roller  at  the  top  contain- 
ing a  strong  spring,  which  is  wound  up  when  the  screen 
is  pulled  down,  —  a  large  curtain  fixture.  A  wooden 
roller  sixteen  feet  long  is  likely  to  sag  in  the  middle, 
unless  it  is  made  so  large  as  to  be  cumbersome.  It  is 
best  to  have  one  made  of  tin  tube  about  three  inches  in 
diameter. 

A  screen  can  quickly  be  put  up  in  any  room  by  pro- 
curing two  strips  of  board,  two  or  three  inches  broad, 
and  long  enough  to  reach  from  the  floor  to  the  ceiling. 
Fasten  the  sides  of  the  screen  to  these,  and  then  wedge 
them  tightly  between  the  floor  and  the  ceiling.  A 
portable  frame  which  can  be  adjusted  to  various  heights 
may  be  made  by  having  two  such  strips  for  each  side  : 
one  of  them  to  be  provided  with  a  collar  at  its  end  for 
the  other  to  slide  through,  and  to  be  made  fast  together 
by  a  thumb-screw  through  the  collar,  as  in  the  figure. 


This  will  permit  one  to  adjust  it  to  different  heights  to 
its  limit  of  eighteen  or  twenty  feet,  while  by  resting  the 
foot  upon  chairs  or  tables  a  still  higher  room  would  be 
provided  for. 


THE  ART  OF  PROJECTING. 


CHAPTER    II. 

ARTIFICIAL    LIGHTS. 

While  it  is  true  that  sun-light  is  much  brighter  than 
artificial  light,  and  is  therefore  very  desirable  as  the  illu- 
minating agent  in  projections,  it  is  also  true  that  sun- 
light is  not  always  to  be  depended  upon,  and  it  will  fre- 
quently disappoint  one,  by  reason  of  clouds,  which  will 
entirely  prevent  using  the  porte  lumiere  and  the  ex- 
periment will  need  to  be  postponed  until  the  sky  is 
again  clear.  In  some  circumstances  such  delay  would 
be  no  serious  matter,  and  one  could  very  well  wait ;  at 
other  times  the  delay  would  be  very  inconvenient  and 
work  some  harm  m  our  educational  institutions  j  hence 
recourse  is  had  to  artificial  light  and  lanterns.  As 
nearly  every  kind  of  projection  is  possible  in  this  way, 
and  some  persons  will  be  provided  with  such  instru- 
ments, and  still  others  who  would  like  to  know  what 
can  be  done  with  lanterns,  some  space  will  be  given  to 
descriptions  of  some  of  their  more  common  forms  and 
their  applications. 

THE   ELECTRIC   LIGHT. 

Chief  among  the  artificial  lights  used  in  projecting 
is  the  electric  light,  which  is  produced  when  a  powerful 
current  of  electricity  is  made  to  pass  between  two 
carbon  points  which  are  separated  a  short  distance  from 
each  other.     It  is  necessary  to  have  a  current  of  elec- 


ARTIFICIAL  LIGHTS  9 

tricity  f i  om  forty  or  more  Grove  or  Bunsen  cells  to  pro- 
duce this  light,  and  there  is  then  needed  a  special  kind 
of  lamp  furnished  with  some  mechanism  that  will  auto- 
matically keep  the  two  carbon  points  at  a  proper  height 
and  a  certain  distance  apart.  Such  lamps  have  been 
devised  by  Dubosque  of  Paris,  and  Browning  of  Lon- 
don, and  others  ;  but  the  best  of  them  are  not  constant, 
except  with  a  very  powerful  battery,  and  when  used 
with  only  forty  or  fifty  cells  will  need  personal  attention 
every  few  minutes.  Browning  has  advertised  a  small 
electric  lamp,  which  he  says  will  give  a  constant  light 
with  only  six  or  eight  cells.  A  number  of  these  small 
lamps  have  been  brought  to  this  country,  but,  so  far  as 
the  writer  knows,  no  one  has  been  able  to  work  them 
with  anything  like  so  small  a  battery. 

There  are  several  reasons  why  the  electric  light  is 
not  more  generally  used  for  this  and  other  purposes. 
First,  its  cost:  the  battery  with  lamps  costing  about 
two  hundred  dollars.  Second,  the  consumption  of  zinc, 
acids,  and  mercury  for  amalgamation,  which,  with  the 
labor  of  setting  it  up  and  cleaning  it  after  use,  may 
be  reckoned  at  ten  dollars  a  day.  Third,  the  noxious 
fumes  which  constantly  arise  from  a  working  battery, 
making  it  necessary  to  have  a  special  battery-room,  well 
ventilated;  and  fourth,  the  need  of  frequently  over- 
hauling it,  re-amalgamatino^  the  zincs  and  filing  the  wire 
connections.  These  have  made  every  one  who  has  ever 
worked  with  a  battery,  wish  that  some  substitute  could 
be  found  for  it.  The  magneto-electric  machines  de- 
vised by  Wilde,  Ladd,  Farmer,  and  others,  have  been 
more  or  less  successful,  but  have  been  much  too  costly, 
and  require  eight  to  ten  horse-power  to  run  them. 

The  machine  that  promises  the  most  for  us  now  is 
the  one  known  as  the  Gramme  machine,  a  French  in- 


lO  THE  ART  OF  PROJECTING. 

vention.  All  who  have  seen  its  performance  speak  in 
high  terms  of  praise  of  it.  At  present  there  is  but  one 
of  them  in  the  United  States  ;  that  one  belongs  to  the 
University  of  Pennsylvania,  at  Philadelphia.  Its  cost 
was  about  $i,ooo.  It  needs  but  one  or  two  horse-power 
to  run  it  eight  hundred  revolutions  a  minute,  when  it 
gives  a  light  equal  to  i,6oo  candles.  The  latest  pattern, 
made  especially  for  produciug  the  electric  light,  weighs 
400  pounds,  is  run  by  one-horse  power,  and  gives  a  light 
equal  to  2,000  wax  candles.  It  would  seem  as  though 
this  was  the  thing  we  have  so  long  waited  for.  It  is 
now  being  used  for  lighting  up  large  manufactories,  only 
four  lamps  being  needed  in  a  room  three  hundred  feet 
long,  which  is  so  well  lighted  as  to  leave  no  shadows. 

MAGNESIUM   LIGHT. 

Wire  made  of  the  metal  magnesium  can  be  lighted 
like  a  piece  of  pine  wood,  and  continues  to  burn 
with  a  most  brilliant  and  dazzling  light.  In  order  to 
regulate  the  burning  of  the  wire  or  narrow  ribbon, 
which  is  generally  employed,  a  lamp  with  a  feed  run  by 
clock-work  is  used.  Sometimes  two  ribbons  are  burned 
at  the  same  time.  This  light  is  not  constant,  and  is 
even  more  liable  to  go  out  than  the  electric  light,  and 
furthermore  a  special  arrangement  of  cloth  tubing  is 
used  for  carrying  away  the  product  of  the  combustion, 
magnesium  oxide,  a  bulky  white  powder,  which  accu- 
mulates very  rapidly. 

The  cost  of  a  lamp  is  about  fifty  dollars,  and  the 
cost  of  the  magnesium  is  about  two  dollars  an  hour. 
It  has  the  great  advantage  of  being  very  compact,  re- 
quiring but  a  few  minutes  to  ])repare  at  any  time,  and 
giving  then  a  light  which  is  amply  sufficient  for  ary  ex- 


ARTIFICIAL  LIGHTS.  II 

hibition,  and  is  especially  well  adapted  for  experiments 
in  fluorescence  on  account  of  the  abundance  of  ultra 
violet  rays.  A  three-inch  transparency  can  be  magni- 
fied up  to  thirty  feet  in  diameter,  and  be  well  enough 
lighted  for  a  large  audience  to  see  plainly, 

THE  OXYHYDROGEN,   OR  DRUMMOND   LIGHT. 

A  very  intense  light  is  produced  by  projecting  a  blow- 
pipe flame  of  mixed  hydrogen  and  oxygen  gases  upon  a 
stick  of  unslacked  lime.  The  great  heat  raises  the 
lime  to  vivid  incandescence.  Sometimes  magnesia  is 
employed  instead  of  lime ;  it  is  then  called  magnesia 
light,  and  when  zirconia  is  used  it  is  called  zirconia  light 
The  two  latter  are  seldom  used  in  the  United  States, 
but  the  former  is  very  common.  The  gases  are  stored 
in  portable  tanks  or  india-rubber  bags,  which  are  con- 
nected by  flexible  tubes  to  the  jet,  from  which  it  is 
driven  by  pressure  upon  the  bags,  and  is  lighted  at  the 
outlet.  There  are  many  patterns  of  these  jets,  some 
of  them  permitting  the  gases  to  mix  before  their  icscape, 
and  others  not  until  they  are  ignited.  The  mixed  jet 
is  the  most  economical  one,  and  is  to  be  preferred  for 
most  purposes.  Such  a  piece  of  apparatus  had  better 
be  bought  of  a  reliable  dealer. 

Common  illuminating  gas  can  be  used  in  place  of 
pure  hydrogen,  but  the  light  is  not  quite  so  intense. 
The  demand  for  these  gases  has  been  so  great  during 
the  past  three  or  four  years  that  they  are  now  manufac- 
tured on  a  commercial  scale  in  New  York  city,  and  are 
compressed  into  copper  tanks  holdmg  from  ten  to  sixty 
cubic  feet.  These  tanks  are  exceedingly  convenient. 
They  retail  for  twenty-two  cents  a  foot  for  oxygen,  and 
eight  cents  a  foot  for  the  common  gas. 


12 


THE  ART  OF  PROJECTING. 


An  alcohol  flame  is  sometimes  used  with  oxygen,  the 
jet  supplying  the  latter  forcing  the  flame  upon  the  piece 
of  lime.  The  alcohol  is  fed  from  a  reservoir  seen  at 
the  back  of  the  lantern.     This  gives  an  excellent  light, 


Fig,  3. 

and  quite  sufficient  for  many  purposes.  A  picture  three 
inches  m  diameter  may  be  magnified  up  to  fifteen  feet 
and  be  well  lighted.  The  light  produced  in  this  way  is 
called  the  Bude  light. 

OTHER   LIGHTS. 

Common  illuminating  gas  may  be  employed  with 
advantage  where  the  room  is  small,  and  great  intensity 
is  not  required.  The  form  of  burner  known  as  the 
Argand,  is  best  for  this  use. 

Kerosene  and  lard   oils   have   been,  and   are   still 


ARTIFICIAL   LIGHTS.  1 3 

largely  employed.  When  burned  under  the  most  favor- 
able conditions,  kerosene  will  yield  a  light  equal  to 
thirty  or  forty  candles,  and  will  illuminate  a  disk  eight 
or  ten  feet  in  diameter  very  well. 

Illuminating  gas  and  kerosene  have  this  advantage, 
that  they  are  very  cheap,  -costing  but  a  few  cents  an 
hour  j  but  they  can  only  be  used  to  enlarge  pictures 
which  are  very  transparent,  outline  drawings,  or  chem- 
ical reactions  in  the  large  tank,  which  will  be  described 
further  on. 

There  is  no  absolute  standard  of  the  luminous  inten- 
sity of  light.  A  conventional  standard  of  its  intensity 
is  used  for  convenience,  and  for  regulating  the  illu- 
minating power  of  gas.  In  Massachusetts,  the  legal 
standard  is  the  light  of  a  sperm  candle  weighing  two 
and  two-thirds  ounces,  when  consuming  one  hundred 
and  twenty  grains  in  an  hour.  In  some  places  wax 
candles  are  adopted,  but  the  light  does  not  differ  much 
in  intensity  from  that  given  by  sperm  candles. 

With  such  a  standard  as  this,  the  various  lights 
which  have  been  specified,  have  the  following  relative 
illuminating  powers  under  favorable  circumstances : 

Illuminating   gas,         .         .         .         .         .         •  ^5 

Coal  oil  in  Argand  burner,      ....  20 

Coal  oil  in  Marcy's  lamp,             ....  25 

Magnesium,              ......  40 

The  lime  light  with  oxygen  and  alcohol,       .         .  50 

*'       *'         '*       "          "          "    common   gas,  too 

"       '*         "       '«          "'          '*    hydrogen,             .  125 

The  Electric  light,  .         .         .         500  to  10,000 

All  of  these  are  very  variable,  especially  the  lime 
light,  which  depends  upon  the  quality  and  pressure  of 
the  gases,  the  quality  of  the  stick  of  lime,  and  the  kind 
of  jet  used. 

The  electric  light  has  a  very  wide  range  in  intensity 


14 


THE  ART  OF  PROJECTING, 


as  will  be  seen.  It  has  not  been  found  possible  yet  to 
make  an  electric  light  with  less  intensity  than  several 
hundred  candle-power,  while  with  the  new  gramme 
machine  before  mentioned,  it  is  expected  that  not  less 
than  twenty  thousand  candle-power  will  be  obtained. 

Sun  light  has  about  four  times  the  intensity  of  the 
most  powerful  electric  light  that  has  yet  been  meas- 
ured. When  it  is  reflected  from  a  good  mirror,  it  loses 
its  brilliancy  somewhat,  but  is  second  to  nothing  but 
the  direct  sun  light.  Hence,  the  desirability  of  em- 
ploying the  latter  whenever  it  is  possible,  for  both  effi- 
ciency and  cheapness. 


LANTERNS 


When  sun  light  is  used,  the  room  must  be  darkened, 
and  the  light  only  admitted  through  the  opening  of  the 
porte  lumiere ;  but  when  artificial  lights  are  used,  it  be- 
comes necessary,  not  only  to  have  the  room  dark,  but 
to  inclose  the  light,  so  that  what  is  not  used  shall  not 
interfere  with  what  is  used.  The  refractive  power  of 
lenses  is  made  available  for  the  purpose  of  securing  a 
larger  amount  of  light  than  is  possible  without.  'J'his 
will  be  understood  by  the  diagram.     Let  a  be  a  pomt 

of  light,  and 
^  <r  an  object 
three  inches 
long  and  dis- 
tant six  or 
eight  inches 
from  the  light. 

Fig.  4. 


LANTERNS. 


15 


All  the  rays  which  will  fall  upon  the  object  will  be  in- 
cluded in  the  angle  b  a  c.  Interpose  the  lens  de  be- 
tween the  light  and  the  object,  and  all  the  rays 
included  in  the  much  greater  angle  d  a  e  will  now 
fall  upon  b  r,  and  it  will  be  much  brighter.  There  is 
always  another  lens,  and  sometimes  two,  called  the 
objective,  in  front  of  the  lantern,  to  give  definition  to  the 

picture.  All  the 
essential  parts 
of  a  lantern  are 
shown  i  n  the 
accompanying 
picture  :  —  «  is 
the  source  of 
light  inclosed  in 
the  box  b  ;  d  e, 
^^(f*  ^'  the  lens  for  di- 

verting a  greater  number  of  light-rays,  and  called  the 
condenser;  c  is  the  objective ;  and  j,  the  screen  to  receive 
the  light. 

The  forms  of  lanterns  differ  somewhat  as  they  are 
adapted  to  different  purposes,  and  they  are  called 
stereopticons  when  the  electric,  the  magnesium,  or  the 
lime  light  is  used  in  them. 

The  electric  lamp  is  generally  placed  within  the  lan- 
tern box,  which  is  made  to  accommodate  either  it,  or 
the  lime  light,  as  is  most  convenient  or  desirable. 

The  following  engraving  (Fig.  7)  represents  the  essen- 
tial conditions  for  the  lime  light  in  a  very  convenient 
form.  Within  the  lantern  may  be  seen  the  fixtures  for 
holding  the  lime  and  the  jet,  with  the  rubber  tubing  to 
connect  with  the  gas  tanks.  The  front  side  and  back 
are  provided  with  heavy  black-cloth  curtains  which  may 


i6 


THE  ART  OF  PROJECTIJVG. 


Fig,  7. 


^^ig,  S, 


easily  be  raised  to  adjust  the  fixtures  within,  yet  shut 
in    the    light    when    allowed  to  hang   free.     The    con- 


ELECTRIC  LIGHTS. 


17 


denser  consists  of  three  lenses,  the  front  one  being  five 
inches  in  diameter.  This  is  shown  in  the  cut  (Fig.  8) 
of  the  transverse  section  of  it.  The  objective  repre- 
sented is  also  compound.  This  form  of  lantern  is 
made  by  Hawkridge  at  the  Stevens  Institute,  Hobo- 
ken,  N.  J.,  and  called  the'  Experimenter's  Lantern,  be- 
cause of  its  adaptability  to  many  kinds  of  experimental 


mg.  9. 


work,  as  well  as  to  the  exhibition  of  photographic  trans- 
parencies. 

Mr.  L.  J.  Marcy,  of  Philadelphia,  has,  in  an  ingen- 
ious manner,  made  a  lantern  jet  which  can  be  used 
with  alcohol,  common  gas,  or  hydrogen,  to  produce  the 
lime  light.  He  calls  it  the  triple  jet.  The  engrav- 
ing (Fig.  9)  shows  the  lantern  in  section.  L  being  the 
disc  of  lime,  the  gases  ignited  at  w.  This  lantern  is 
compact,  light,  and  has  a  very  convenient  arrangement 
for  holding  slides,  tanks,  and  so  forth. 


i8 


THE  ART  OF  PROJECTING. 


The  oil  lanterns  have  various  names  given  to  them 
by  different  makers,  such  as  magic  lanterns,  lampo- 
scopes,  sciopticons,  and  so  on.  Fig.  lo  represents 
Marcy's  Sciopticon.  On  the  opposite  page  are  cuts  of 
an  oil  lantern,  and  an  oxyhydrogen  lantern  with  micro- 


mg.  10. 


scope  attachment  for  physical  experiments,  both  by 
Queen  &  Co.;  the  latter  one  is  called  a  physical  pro- 
jector^ and  it  is  adapted  to  a  wide  range  of  work.  But 
little  time  will  be  needed  to  learn  all  that  is  needful  to 


LENSES.  19 

know  in  order  to  work  any  of  them  well,  and  they  are 
not  likely  to  become  disarranged. 

The  course  of  experiments  to  be  given  will  be  gen- 
erally adapted  to  both  the  parte  lumiere,  and  the  lan- 
tern, but  the  adjustments  will  be  described  for  the 
former.  If,  however,  some  special  arrangement  of  the 
lantern  will  be  needed  for  a  given  experiment,  it  will 
be  pointed  out. 

A  sufficient  number  and  variety  of  experiments  in 
physics,  chemistry,  and  natural  history,  will  be  de- 
scribed, to  make  any  one  who  is  practically  interested 
in  the  work  so  familiar  with  the  apparatus  and  its 
working,  that  he  will  need  no  further  instruction  in  the 
art  of  projecting. 


LENSES. 

All  the  lanterns  in  the  market  are  furnished  with  the 
proper  lenses  for  the  ordinary  kinds  of  projections, 
such  as  transparencies,  etc.  Solar  microscopes  are  also 
generally  well  provided  with  ler.ses  suitable  for  the  work 
to  be  done  with  them,  but  as  this  treatise  is  for  the  use  of 
those  who  have  neither,  yet  who  would  like  to  experi- 
ment, some  things,  necessary  to  be  knowrj  about  lenses, 
and  their  uses  are  mentioned  here,  chiefly  as  to  their 
adaptability  to  the  experiments  with  the  porte  lumiere. 
In  Fig.  1 1  the  rays  of  light  are  shown  to  fall  upon  the 


20 


THE  ART  OF  PROJECTING. 


Fig,  11. 
fall  upon  the  screen  s. 


double  convex 
lens  cd  when 
they  are  con- 
verged to  a 
focus,  and  aft- 
erward they 
separate  and 
It  may  be  noticed  that  this 
lens,  used  next  to  the  orifice,  is  represented  as  a 
double  convex  lens  in  nearly  every  place  in  the  book. 
Such  a  lens,  when  properly  constructed,  has  less 
spherical  aberration  than  any  other  form.  To  be  thus 
properly  constructed  one  side  should  have  a  greater 
convexity  than  the  other,  the  radii  of  curvature  for 
the  two  sides  being  as  one  to  six,  but  such  lenses  are 
not  enough  better  than  the  more  common  form  of  hav- 
ing the  two  sides  of  equal  convexity,  to  make  any  dif- 
ference except  for  special  work,  if  the  lens  is  used  as  a 
condenser :  so  that  for  this  purpose  a  common  double 
convex  lens,  with  the  faces  of  equal  curvature,  will  be 
found  to  answer  for  most  purposes.  But  a  plano-con- 
vex lens  with  the  same  focal  length  as  the  convex  lens 
is  nearly  as  good. 

Here  it  may  be  remarked  that  whenever  a  plano- 
convex lens  is  used  it 
should  have  its  convex 
side  turned  toward  the 
rays  which  are  parallel, 
or  most  nearly  parallel. 
Thus  the  lens  (Fig.  12) 
has  its  convex  side  to- 
ward a,  where  the  rays 
are  parallel,  and  should 
Fig,  12*  be  so  whether  the  rays 


LENSES.  21 

enter  that  side  or  emerge  from  it,  when  the  source 
of  light  is  at  h.  It  must  not  be  inferred  that  noth- 
ing can  be  done  except  with  lenses  of  a  particular 
sort.  On  the  contrary  very  much  can  be  done  with 
such  poor  lenses  as  are  used  in  dark  lanterns,  and 
are  full  of  striae  and  air  bubbles.  The  lenses  that 
come  with  ordinary  magic  lanterns  will  answer  for  many 
purposes.  Spectacle  glasses,  linen  provers,  botanical 
glasses,  are  all  very  useful.  A  pow- 
erful lens  can  be  made  out  of  two 
watch  glasses,  one  {a)  a  little  larger 
than  the  other  {b).  Bring  the  two 
together  under  clear  water.  When 
raised  out  of  the  water  they  will 
adhere  quite  strongly,  and  for  a  time 
can  be  used  to  advantage  as  a  mag- 
nifier. 

FOCAL   LENGTH. 

mg.  13.  The  focal  length  of  a  lens  should 

be  known  before  it  is  brought  into  use,  and  it  may  be 
determined  experimentally  in  the  following  way  : 

With  the  lens  placed  as  shown  in  Fig.  12,  so  that  the 
parallel  rays  from  the  porte  lumiere  fall  squarely  upon 
it,  measure  the  distance  from  the  centre  of  the  lens  to 
the  point  b,  the  focus.  If  the  lens  be  double-convex, 
add  one-half  its  thickness  to  the  measured  line.  This 
number  will  represent  the  focal  length  of  the  lens.  If 
the  lens  be  plano-convex,  its  focal  length  will  be  the 
distance  from  its  flat  side  (Fig.  12)  to  the  focus.  Again, 
hold  the  lens  so  that  the  direct  rays  of  the  sun  fall  per- 
pendicularly upon  it,  and  measure  as  before  the  dis- 
tance to  the  focus.  Lastly,  if  the  sun  be  not  shining, 
bring  the  lens  close  to  a  white  wall  or  sheet  of  paper 
opposite  to  a  window,  and  hold  it  so  that  the  light  from 


22  THE  ART  OF  PROJECTING. 

the  window  falls  perpendicularly  upon  it ;  then  slowly 
move  the  lens  from  the  paper  toward  the  window,  until 
the  inverted  image  of  some  object  out  of  doors,  such  as 
a  cloud,  or  house,  or  tree,  half  a  mile  or  more  away, 
appears  plainest.  Measure  the  distance  from  the  lens 
to  the  paper,  as  in  the  other  cases. 

If  diverging  rays  had  been  used  instead  of  parallel 
rays,  the  focus  would  have  been  at  some  greater  dis- 
tance ;  and  if  converging  rays,  a  less  distance  than  that 
indicated  for  parallel  rays,  and  these  differing  foci  may 
be  infinite  in  number ;  hence,  the  focal  length  of  a  lens 
is  always  specified  for  parallel  rays. 

CARE   OF   LENSES. 

Before  using  lenses,  or  other  pieces  of  glass  appara- 
tus, see  to  it  that  they  are  clean,  and,  as  far  as  possible, 
avoid  touching  them  upon  their  polished  surfaces  with 
the  fingers,  as  the  latter  will  always  leave  a  mark  upon 
such  a  surface.  A  piece  of  cotton-flannel,  or  old  fluffy 
linen  will  do  to  wipe  glasses  with.  Wet  the  cloth  with 
water,  or,  still  better,  with  alcohol,  if  there  are  spots 
that  will  not  otherwise  come  off.  One  may  know  when 
a  piece  of  glass  is  clean  by  gently  breathing  upon  it, 
and  then  noticing  how  long  the  condensed  moisture 
remains  upon  its  surface.  If  it  is  really  clean,  the 
moisture  will  disappear  in  a  second  or  two  ;  if  it 
remains  for  eight  or  ten  seconds  or  longer,  the  glass  is 
not  clean,  though  it  may  appear  to  be  so  to  the  eye. 
Be  careful  to  keep  such  pieces  from  touching  anything 
harder  than  the  cloth  they  are  wiped  with,  for  even 
wood  will  scratch  a  nicely  polished  glass  surface.  If 
these  pieces  are  not  mounted  in  such  a  way  as  to  pro- 
tect them,  they  should  be  laid,  when  not  in  use,  upon  a 
piece  of  cotton-flannel  or  velvet. 


LENSES.  23 

MOUNTINGS    FOR    LENSES. 

Lenses  may  be  purchased  already  mounted  in  any 
desirable  way,  but  generally  the  mounting  costs  p.s  much 
or  more  than  the  lens.  What  is 
needed  is  a  fixture  that  will  hold 
the  lens  at  a  proper  height,  and 
has  a  considerable  latitude  of 
movement  in  every  direction. 
For  many  purposes  an  unmount- 
ed lens  can  be  held  in  the  pinch 
of  a  common  retort-holder  as  in 
Fig.  14. 

^*^  ^  *  It    is    often    convenient,    and 

sometimes  necessary,  to  have  the  lens  so  mounted  as 
to  cut  off  light  that  would  otherwise  pass  by  its  edge 
and  mar  the  effect  upon  the  screen.  To  do  this  it  is 
only  needful  to  cut  a  hole  the  size  of  the  lens,  in  a 
square  piece  of  board  (Fig.  15)  having  the  proper 
size  and  thickness,  and  fasten 
the  lens  m  it  with  small  brads 
or  triangular  pieces  of  tin  or 
zinc,  such  as  glaziefs  use  for  fast- 
enmg  window-glass  in  the  sash. 
The  board  should  be  thicker  than 
the  lens,  and  the  latter  should  be 
Fig.  15.  SQ  sunk  into  it  that  its  surface  will 

not  touch  the  table  when  lying  upon  it.  This  will  pre- 
vent it  from  getting  scratched.  Thus  mounted,  the 
board  may  be  held  in  the  retort-holder  as  before. 

The  smaller  lenses,  having  a  focal  length  of  not  more 
than  an  inch  or  two,  had  better  be  bought  already 
mounted  for  carrying  in  the  pocket,  such  as  botanical 
glasses  and  linen  provers,  or,  if  it  can  be  afforded,  get 
one  of  Zentmayer's  gas  microscope  objectives. 


.24 


ThE  ART  OF  PROJECTING. 


For  holding  pictures,  and  other  objects,  for  projec- 
tion, the  retort-holder  will  answer,  in  many  cases,  just 
as  well  as  a  more  costly  fixture.  Not  only  may  the 
larger  pieces,  such  as  photographic  transparencies, 
leaves  of  trees,  and  the  like,  be  held  well  in  it,  but 
microscopic  specimens  in  glass  slides,  also  small  un- 
mounted objects,  such  as  parts  of  flowers,  insects,  etc., 

may  be  held  in 

small   forceps 

^^'  ^«-  (Fig.  i6),  which 

in  turn  may  be  held  in  the  retort-holder.     It  will  be 

found   convenient   to  have  as   many   as   three  of   the 

latter. 


PROJECTIONS. 


TO     PROJECT    WITH     THE     PORTE     LUMIERE    AND    A 
SINGLE     LENS. 

Fasten  t\\Qj>orte  himiere  in  its  place,  and  so  adjust  it 
that  the  beam  of  light  /  (Fig.  17)  is  reflected  horizon- 
tally, and  falls  upon  the  screen  s.     It  will  appear  as 

a  bright  spot, 
five  or  six  inch- 
es in  diameter. 
Darken  the 
room,  by  draw- 
ing the  curtains 
or  closing  the 
shutters,  and 
the  beam  of 
:E'ig.  It,  light  can  then 


PROJECTIONS.  25 

be  seen  from  the  window  to  the  screen  by  the  light  re- 
flected from  the  dust  particles,  which  are  always  in  the 
air.  Now  fasten  in  the  retort-stand  a  lens  o  four  or  five 
inches  in  diameter,  and  with  a  focus  of  a  foot  or  more, 
and  place  it  two  or  three  feet  from  the  opening,  in  the 
path  of  the  beam,  and  perpendicular  to  it.  It  will  at 
once  be  noticed  that  the  light  is  converged  by  the  lens, 
the  rays  crossing  each  other  in  front  of  it,  at  its  focus, 
from  which  they  diverge,  and  appear  upon  the  screen  as 
a  large  disk  of  light.  If  some  object,  as  d^  (Fig.  17), 
be  placed  between  the  opening  and  the  lens  0,  a  place 
may  be  found  by  trial,  when  the  image  of  the  object 
will  be  seen  upon  the  screen.  The  outline  should  be 
well  defined  ;  it  will  be  inverted  and  much  enlarged. 

Finding  the  right  adjustment  of  the  object  and  the 
lens,  so  that  the  image  is  in  its  proper  place,  and  has  a 
sharp  outline,  is  called  focusing  In  general  it  is  best 
done  by  fixing  the  object  in  the  path  of  the  beam  first, 
and  then  placing  the  lens  rather  close  to  it,  and  slowly 
moving  the  lens  toward  the  screen,  being  careful  to  keep 
it  perpendicular  to  the  beam  until  the  image  is  plainest. 
It  will  be  well  for  a  beginner  to  take  a  number  of  ob- 
jects :  some  opaque,  like  the  finger,  a  pencil,  or  a  key, 
and  some  transparent,  as  a  grasshopper*s  wing,  or  a 
piece  of  glass  with  a  design  drawn  upon  it,  or  a  regu- 
lar lantern  transparency.  A  lens  thus  used  to  project 
a  picture  is  called  an  objective. 

These  two  pieces  of  apparatus,  \.h&  porte  iumiere  and 
the  single  lens,  have  a  much  wider  application  than  one 
unfamiliar  with  them  might  suppose.  Every  picture 
made  for  the  magic  lantern,  or  the  stereopticon,  can  be 
shown  with  these  in  the  day-time,  even  better  than  with 
the  others  at  night.  Every  school  in  the  land  may  have 
one,  for  the  carpenter  can  make  the  porte  lumiere^  and 


t6  THE  ART  OF  PROJECTING. 

the  lens  will  cost  but  a  trifle.  The  pictures  themselves, 
though  not  half  as  costly  as  they  were  before  photogra- 
phy was  applied  in  making  them,  can  be  rented  of  any 
one  who  keeps  them  for  sale,  if  one  cannot  afford  to 
buy  them  outright.  Most  excellent  transparencies,  on 
all  sorts  of  subjects,  can  be  bought,  from  six  to  nine 
dollars  a  dozen,  of  any  lantern-maker  or  dealer  in  pho- 
tographic materials. 

If  the  teacher  wished  to  give  a  lesson  on  the  elements 
of  drawing,  his  copies  could  be  prepared  upon  glass,  by 
one  of  the  methods  given  a  little  further  on.  These,  when 
projected,  would  be  so  large  that  a  large  school  could  see 
them  as  plainly  as  if  they  had  been  drawn  upon  a  huge 
blackboard,  with  chalk.  The  room  could  be  light 
enough  for  any  of  the  required  work.  Geometrical  fig- 
ures, outline  maps,  botanical  specimens,  the  kaleido- 
scope, chemical  reactions  in  a  large  test  tube ;  natural 
history  specimens,  such  as  small  fish,  pollywogs,  water 
beetles,  butterflies,  grasshoppers  ;  the  splendid  colors 
on  huge  soap-bubbles ;  the  vibrations  of  tuning-forks, 
and  of  cords ;  the  intensity  of  light,  reflection,  refrac- 
tion, magnifying  power  of  lenses,  and  many  more  things, 
may  be  projected,  in  an  admirable  way,  with  only  these 
two  pieces. 

THE   CONDENSER  AND   ITS   USE. 

The  rays  of  light  reflected  from  the  mirror  a  (Fig.  17) 
through  the  aperture,  are  parallel,  and  the  diameter  of 
the  lens  0  should  be  as  great  as  the  thickness  of  the 
beam,  and  it  may  have  a  greater  diameter  with  advan- 
tage. If  it  has  less,  some  of  the  light  will  pass  its 
edge,  and  either  be  unused  or,  what  would  be  worse, 
fall  upon  the  screen  and  make  a  bright  spot  in  the  mid- 
dle of  the  picture.     The  smaller  the  object  to  be  pro- 


POOJECTIONS.  27 

jected,  the  smaller  must  be  the  lens  used  as  the  object- 
ive, and  the  shorter  must  be  its  focal  length  \  hence,  if 
a  beam  of  parallel  rays  is  used,  it  may  often  be  so  small 
as  to  be  nearly  useless,  for  the  divergence  is  so  rapid 
beyond  the  focus  of  a  short  focus  lens  that  the  little 
light  thus  used  would  be  too  much  scattered.  It  be- 
comes necessary,  then,  to  make  a  large  beam  of  light 
to  pass  through  the  small  lens.  This  is  accomplished 
by  means  of  a  sec'jnd  lens,  called  the  condenser^  because 
its  office  is  to  condense  a  large  number  of  light  rays  for 
the  double  purpose  of  illuminating  the  object  better 
and  making  them  all  to  pass  through  the  smaller  lens. 
This  condenser  is  usually  four  or  five  inches  in  diameter, 
though  for  special  purposes  it  is  sometimes  made  a  foot 
or  more  in  diameter.  For  the  porte  lu77iiere  the  con- 
denser may  be  the  same  lens  that  was  used  as  the  ob- 
jective, or  any  lens  may  be  used  that  has  a  sufficient  di- 
ameter and  a  focus  of  from  one  to  two  feet.  It  may  be 
either  double-convex  or  plano-convex. 

TO    PROJECT   WITH    A    CONDENSER. 

The  object  ^  (Fig.  18)  is  placed  near  the  condenser 
c,  and  the  objective  0  is  brought  near  to  it  and  slowly 

moved  toward 
the  screen,  as 
before,  until 
the  well  de- 
fined image  ap- 
pears upon  it. 
It  must  be  not- 
Fig.  18.  ed    here    that 

the  size  and  focal  length  of  the  objective  must  be  such 
that  all  the  light  passes  through  it  when  it  is  at  its 
proper  distance  from  the  object.     If  ^  be  moved  toward 


28  THE  ART  OF  PROJECTING. 

the  object,  it  will  be  seen  that  some  of  the  light  does 
not  pass  through  it.  If  the  object  d  be  moved  toward 
the  objective,  then  some  parts  of  it  will  not  be  lighted, 
and  consequently  but  a  part  of  it  would  be  projected. 
If  the  object  d  is  quite  small,  like  a  fly,  or  a  flea,  or  a 
small  crystal,  it  will  be  necessary  to  bring  it  forward, 
toward  the  focus  of  the  condenser,  where  it  will  be 
more  strongly  lighted,  and  allow  the  use  of  an  objective 
of  shorter  focus,  and  consequently  higher  magnifying 
power.  If  the  object  be  made  of  wood,  or  any  kind  of 
tissue,  be  careful  about  bringing  it  very  near  to  the 
focus,  as  the  great  heat  there  may  destroy  it  in  a  few 
seconds.  This  danger  may  be  somewhat  lessened  by 
placing  between  the  condenser  and  the  object  the  chem- 
ical tank,  containing  a  strong  solution  of  alum.  The 
common  pocket  botanical  glass,  having  a  focus  of  an 
inch  or  two,  will  answer  for  very  much  of  this  work, 
but  Zentmayer*s  inch-and-a-half  gas  microscope  object- 
ive is  superior  to  any  other  lens  I  have  seen  for  such 
projections. 

This  arrangement  is  essentially  the  solar  microscope^ 
The  object  may  be  exceedingly  minute  if  the  objective 
has  a  very  short  focus,  say  half  an  inch,  or  less. 

It  is  possible  to  magnify  an  object  a  thousand  diam- 
eters, or  a  million  times,  and  still  have  it  so  well  lighted 
that  a  large  audience  can  see  it  plainly.  A  list  of 
things  that  are  suitable  for  projections  with  this  ar- 
rangement is  appended,  mainly  for  the  purpose  of  indi- 
cating the  breadth  of  its  field  of  usefulness : 

Hairs  of  various  animals,  which  may  be  held  between 
two  strips  of  glass.  Down  from  the  wings  of  moths  and 
butterflies ;  these  will  adhere  to  a  piece  of  glass  with- 
out any  pressure.  Scales  of  fishes.  Eyes,  legs,  and 
wings  of  flies,  or  the  whole  of  any  insect.     Stings  of- 


PROJECTIONS.  29 

bees  and  wasps.  Antennae  of  moths  and  mosquitoes. 
Fibres  of  cotton,  woolen,  silk,  etc.  Ferns,  moss,  lichens, 
leaves  of  trees.  Thin  sections  of  wood.  Small  flowers, 
stamens  and  pistils,  pollen.  Mites  in  cheese.  Butter- 
flies, beetles,  animalcules  in  stagnant  water.  Vinegar 
eels.  Crystallization  of  camphor,  sulphate  of  copper, 
and  most  solutions  of  crystallizable  substances. 

Diatoms,  and  indeed  most  objects  that  are  prepared 
for  the  microscope,  appear  to  good  advantage  upon  a 
screen.  Any  book  upon  the  microscope  will  have 
many  valuable  hints  upon  obtaining  and  preparing  ob- 
jects in  a  suitable  way,  and  will  be  a  very  useful  book 
to  one  interested  in  natural  history  but  who  cannot  af- 
ford to  buy  a  good  microscope. 

OUTLINE   DRAWINGS   FOR  PROJECTION. 

Every  one  who  uses  either  a  lantern  or  the  porU 
lumiere  for  purposes  of  instruction,  will  need  to  make 
outline  pictures  to  illustrate  his  subject,  as  it  will  be  fre- 
quently impracticable  to  get  a  photograph  of  what  is 
wanted.  Moreover,  a  simple  outline  is  often  quite  suf- 
ficient for  the  illustration,  as,  for  instance,  superposi- 
tion and  inclination  of  strata  in  geology ;  sections  of 
machines ;  writing,  or  musical  notes  ;  outlines  of  leaves, 
roots,  parts  of  a  flower,  insects,  maps,  etc. 

The  surface  of  transparent  glass  is  so  smooth  that  it 
cannot  be  marked  with  either  common  ink  or  a  lead 
pencil.  If  the  glass  be  ground,  so  that  a  pencil  will 
mark  it,  it  becomes  so  opaque  that  but  little  light  can 
go  through  it ;  hence,  a  surface  must  be  prepared  which 
will  be  transparent  and  yet  allow  marking  upon  it. 
This  can  be  effected  in  many  ways,  and  I  give  a  num- 
ber which  I  know  to  be  practicable : 

I.  If  a  piece  of  glass  be  rubbed  on  one  surface  with 


30  THE  ART  OF  PROJECTING. 

a  piece  of  hard  soap,  enough  will  adhere  to  it  to  make 
the  glass  semi-opaque.  Now  draw  the  design  with  a 
fine-pointed  stick.  It  will  clear  the  soap  from  the  glass, 
and  so  permit  the  light  to  shine  through  the  marks. 
This  has  the  advantage  of  permitting  the  same  glass  to 
be  used  like  a  slate,  for  with  a  drop  of  water  upon  the 
finger  the  old  design  can  be  rubbed  out,  leaving  the 
glass  coated  for  another  picture.  The  same  thing  can 
be  done  with  a  surface  of  beeswax,  but  the  glass  would 
need  heating  in  order  to  re-spread  the  wax. 

2.  For  more  permanent  pictures,  a  very  good  way  is 
to  flow  the  glass  with  photographers*  transparent  var- 
nish, and  then  scratch  the  design  upon  the  varnish,  not 
cutting  through  to  the  glass.  The  light  is  so  much 
scattered  from  this  scratched  surface,  that  it  appears  as 
a  dark  line,  and  answers  a  very  good  purpose.  The 
prepared  plate  can  be  laid  over  the  design  wanted  if  it 
is  to  be  a  copy,  and  is  of  proper  size  ;  the  transparency 
allows  the  picture  to  be  plainly  seen,  and  all  its  mark- 
ings can  easily  be  followed.  The  varnish  is  quite  hard 
when  dry,  and  with  a  little  care  in  handling  these  pic- 
tures, they  need  not  become  scratched.  They  can  be 
entirely  protected  from  that  danger  by  covering  them 
with  another  clean  glass  of  the  same  size,  and  binding 
their  edges  with  paper,  as  common  lantern-pictures  are 
bound.  Photographers  have  also  another  kind  of  var- 
nish called  ground-glass  varnish,  which,  when  spread 
upon  glass,  gives  it  an  appearance  similar  to  ground 
glass.  This  surface  permits  drawing  with  a  pencil  or 
with  ink  upon  it,  and  then  a  coat  of  the  transparent 
varnish  will  render  the  first  coat  transparent,  leaving 
the  lines  in  ink  or  pencil ;  or  the  design  may  be  drawn 
through  the  first  coat  of  varnish,  in  which  case,  the  light 
will  shine  through  the  lines  and  appear  white  upon  the 
screen. 


PROJECTIONS.  31 

3.  If  India  ink  be  rubbed  up  in  water  until  it  is  quite 
thick,  it  can  be  used  for  drawing  designs  upon  ordinary 
glass. 

4.  A  thin  sheet  of  gelatin  may  be  treated  like  the 
glass  coated  with  the  transparent  varnish,  and  either 
have  the  design  scratched  upon  it,  or  drawn  with  ink. 
It  should  be  inclosed  between  two  glasses  for  protection, 

5.  Thin  sheets  of  transparent  mica  will  receive  lines 
drawn  with  india  ink,  or  the  figures  may  be  scratched 
upon  them  with  the  needle  or  awl. 

6.  Designs  may  be  nicely  etched  upon  glass,  by  first 
coating  the  glass  with  a  thin,  even  coat  of  beeswax, 
which  can  be  well  done  by  heating  the  glass  over  a 
lamp  until  the  wax  melts  and  flows  over  its  upper  sur- 
face. When  it  is  cool,  draw  the  design  with  a  needle 
point  or  a  small  awl,  cutting  through  the  wax  all  the 
way.  Take  an  old  saucer,  or  some  such  dish  which  you 
are  willing  to  spoil  for  other  use,  and  put  into  it  a  table 
spoonfull  of  powdered  fluor  spar.  Upon  that  pour  a 
table  spoonful  of  strong  sulphuric  acid,  and  stir  them 
together  with  a  stick.  Fasten  the  glass,  drawing  upper- 
most, to  a  piece  of  board  large  enough  to  completely 
cover  the  dish.  The  fastening  can  be  done  by  crowd- 
ing tacks  into  the  wood,  so  that  the  heads  shall  lap  the 
glass  and  keep  it  in  its  place.  When  thus  fixed  and 
laid  over  the  mixture  of  spar  and  acid,  gently  heat  the 
dish,  being  careful  not  to  inhale  the  fumes  that  will 
escape.  When  the  fumes  begin  to  appear,  put  the 
whole  either  out  of  doors  or  in  a  good  chimney  draught, 
and  let  it  remain  eight  or  ten  minutes,  when  the  wax 
may  be  removed  by  heat  and  rubbing,  and  the  drawing 
will  be  found  etched  into  the  glass. 

Beautiful  pictures  of  crystals  can  be  made  in  this 
way,  by  taking  various  crystallizable  salts,  such  as  am- 


J2  THE  ART  OF  PROJECTINu. 

wonium  chloride,  cupric  sulphate,  etc.,  and  making  a 
rather  dilute  solution  of  them,  and  then  adding  a  little 
dissolved  gum  arable.  Flow  the  solution  over  the 
plate,  and  let  it  remain  horizontal  until  it  is  dry.  The 
crystals  will  be  seen  to  have  separated  from  the  gum, 
which  will  fill  up  all  the  mtermediate  space.  Put  over 
the  etching  dish  as  before.  The  crystals  will  quickly 
dissolve,  and  their  outlines  will  be  beautifully  etched 
upon  the  glass,  which  may  now  be  washed  clean  in 
water. 

7.  Engravings  may  be  transferred  to  glass  by  first 
coating  the  glass  with  dammar  varnish,  or  with  Canada 
balsam,  and  letting  it  dry  until  it  is  very  sticky,  which 
will  take  half  a  day  or  more.  The  picture  to  be  trans- 
ferred should  be  well  soaked  in  soft  water,  and  care- 
fully laid  upon  the  prepared  glass,  and  pressed  upon  it, 
so  that  no  air  bubbles  or  drops  of  water  are  seen  un- 
derneath. This  should  dry  a  whole  day  before  it  is 
touched  \  then  with  the  wetted  finger,  begin  to  rub  off 
the  paper  at  the  back.  If  this  be  skillfully  done, 
almost  the  whole  of  the  paper  can  be  removed,  leaving 
simply  the  ink  upon  the  varnish.  When  the  paper  has 
been  removed,  another  coat  of  varnish  will  serve  to 
make  the  whole  more  transparent. 

8.  A  piece  of  glass  may  be  smoked  in  the  ordinary 
way,  and  a  design  marked  upon  it.  This  makes  a  very 
good  and  plain  picture.  If  the  design  is  needed  for 
keeping,  heat  some  alcohol  in  a  cup  or  small  porcelain 
dish,  and  hold  the  smoked  side  of  the  glass  in  the  alco- 
hol vapor  for  a  minute  or  two,  and  afterward  it  may  be 
varnished  with  photographers'  varnish,  carefully  flowing 
it  over  the  plate  in  the  same  way  that  plates  are  flowed 
Cor  photographic  purposes. 


► 


PROJECTIONS.  33 

TO    DETERMINE   THE    MAGNIFYING    POWER  OF  A  LENS,  OR 
A    COMBINATION    OF    LENSES,    IN    PROJECTING. 

It  will  be  evident,  upon  inspection,  that  the  farther 
away  the  screen  is  from  the  lens,  the  larger  will  be  the 
picture  ;  but  for  a  given  projection,  the  simplest  way  of 
determining  the  magnification  is  to  choose  some  object 
of  known  dimensions  for  projection,  and  then  to  meas- 
ure its  size  upon  the  screen.  Suppose  it  be  a  lead 
pencil  having  a  diameter  of  one  fourth  of  an  inch.  If 
its  image  is  a  foot  in  diameter,  it  is  evident  that  it  is 
magnified  4  X  12  =48  diameters.  If  it  is  three  feet  in 
diameter,  then  it  has  been  magnified  4  X  12  X  3  =  144 
diameters.  It  will  be  convenient  to  have  a  scale,  either 
photographed  or  etched  upon  glass,  for  the  purpose  of 
directly  showing  the  magnifying  power  of  lenses. 

A  ver7iier  made  upon  glass  by  either  of  the  described 
methods,  will  be  convenient  for  study,  and  some  meas- 
urements. 

THE    ANIMALCULE    CAGE. 

If  one  would  exhibit  the  minute  forms  of  life  to  be 
seen  in  water,  an  animalcule  cage  will  be  needed. 
This  may  be  made  in  the  following  way: 

Take  two  quite  clear  pieces  of  white  glass,  about  four 
inches  long  and  one  inch  wide.     Two  other  pieces  of 

the  same  width, 
and  one  inch  and 
a  half  long.  Put 
these  two  short- 
jiig,  19,  er  pieces  be- 

tween the  longer  ones,  so  as  to  separate  them,  and 
leave  a  space  in  the  middle  clear  through.  Fasten 
these  together  with  japan  varnish,  being  careful  not  to 
get  any  of  the  varnish  into  the  opening.     If  any  should 


34 


THE  ART  OF  PROJECTING. 


get  in,  wipe  it  carefully  out.  When  the  varnish  is 
dry,  and  the  pieces  are  firmly  fixed  together,  putty  up 
the  bottom  of  this  hole  so  that  it  will  hold  water. 
When  this  is  dry,  it  can  be  used  to  hold  fluids  of  most 
kinds,  but  it  is  especially  fitted  for  water  containing  ani- 
malcules, or  vinegar  with  eels.  It  should  be  put  back 
of  the  focus  of  the  condenser,  for  the  great  heat  there 
will  boil  the  water  in  a  little  while,  and  the  temperature 
of  no  more  than  130°  Fah.  will  quickly  kill  most  all 
kinds  of  infusoria.  Suitable  water  for  examination  can 
be  found  in  old  rain  barrels,  stagnant  pools,  water  in 
which  flowers  have  been  standing  for  a  day  or  two,  an 
infusion  of  hay  in  water,  and  will  be  found  very  interest- 
ing. The  larva  of  the  mosquito  is  a  lively  and  amus- 
ing thing  when  magnified  to  five  or  six  feet  in  length. 

THE    CHEMICAL    TANK. 

For  chemical  experiments,  and  a  variety  of  others, 
a  tank  of  larger  proportions  will  be  necessary.  The 
accompanying  diagram  (Fig.  20),  shows  the  construc- 
tion. Provide  two  pieces  of  clear,  white  glass,  of  the 
same  size,  about  five  inches  by  six,  for  the  sides.  These 
may  be  kept  apart  by  a  strip  of  rubber,  about  one-half 

of  an  inch  thick, 
bent  and  cut  at 
the  corners,  the 
whole  clamped 
together  by  three 
or  four  clamps, 
as  shown.  If  rub- 
ber with  flat  sides 
is  not  easily  pro- 
curable, a  piece 
of  rubber  t  u  b  - 
Fig,  20.  ing   will    answer 


f 


PROJECTIONS,  35 

nearly  as  well ;  the  tubing  may  be  filled  with  sand  to 
keep  it  firm.  Such  a  tank  will  hold  any  kind  of  a  so- 
lution, and  may  be  quickly  taken  apart  and  cleaned. 
A  tank  which  will  answer  for  many  experiments  nearly 
as  well  can  be  made  by  cutting  a  semi-circular  piece  out 
of  a  board,  of  the  proper  size,  and  fastening  the  glass 
sides  to  it  with  cement.  What  is  known  as  marine  glue 
will  be  the  best  for  this  purpose,  and  as  it  is  very 
convenient  to  have  some  of  this  glue  for  making 
and  mending  apparatus,  because  it  will  adhere  to  any 
surface,  the  method  of  preparing  it  is  given  :  Dissolve, 
separately,  equal  parts  of  shellac  and  India  rubber  in 
naptha,  and  afterwards  mix  the  solutions  thoroughly, 
applying  heat.  It  may  be  made  thinner  by  adding 
more  naptha.  It  may  be  preserved  in  a  tin  box.  In 
order  to  use  it,  it  must  be  heated,  as  well  as  the  sur- 
faces which  are  to  receive  it.  Marine  glue  may  be  dis- 
solved in  ether,  or  a  solution  of  potash. 

A  METHOD  FOR  PROJECTING  LARGE  PIECES  OF  APP/-:^ATUS. 

Many  pieces  of  apparatus  used  in  illustration  and 
demonstration  are  much  too  large  to  be  projected  in  the 
ordinary  way,  as  it  is  obvious  that  the  size  of  the  lens 
used  as  condenser  will  be  the  limit  to  the  size  of  the 
object  that  can  be  shown  with  it.  Thus,  if  sunlight  is 
used,  the  diameter  of  the  orifice  c,  ^/(Fig.  ii),  will  be 
the  measure  of  the  largest  picture  that  can  be  shown  at 
once  ;  and  if  a  lantern  is  employed,  no  picture  larger 
than  the  condenser  can  be  projected. 

Suppose  that  it  is  desirable  to  show  to  an  audience 
a  piece  of  apparatus  much  too  large  for  ordinary  pro- 
jection, and  yet  too  small  to  be  plainly  seen,  such,  for 
instance,  as  the  electroscope  ;  or  the  movement  of  a 
pith-ball  under  electrical  excitement ;  or  the  movement 


36  THE  ART  OF  PROJECTING. 

of  a  vibrating  cord,  or  large  tuning-fork  ;  or  the  ap- 
paratus for  showing  the  linear  expansion  of  metallic 
rods,  etc.  The  following  method  will  be  found  applica- 
ble to  a  great  many  such  cases,  where  simply  the  outline 
of  the  instrument  is  needed. 

Place  a  short  focus  objective  (and  the  shorter  the 
better),  so  near  the  focus  of  the  condenser  that  all  the 
light  falls  upon  it.  After  refraction  the  light  will  form 
a  very  divergent  beam  and  the  focus  in  front  of  o  will 


Fig.  21. 

be  a  sharp  point,  practically  a  luminous  point,  and  any 
object  held  between  it  and  the  screen  j,  will  have  a 
strong  shadow  cast  upon  the  latter.  The  magnitude  of 
this  shadow  will  depend  upon  the  distance  from  the 
focus.  There  will  be  no  penumbra — the  outline  will  be 
sharply  defined. 

If  one  has  a  lantern,  the  condensing  lens  above  will 
answer  without  the  objective,  as  its  focus  for  parallel 
rays  will  be  sufficiently  short.  A  globular  glass  flask, 
filled  with  water  and  placed  in  the  path  of  the  rays,  will 
also  be  found  to  be  satisfactory.  When  a  lantern  is 
used  instead  of  sunlight,  it  will  be  necessary  to  use  the 
microscope  attachment,  which  is  described  further  on. 


PROJECTIONS.  37 

working  in  front  of  the  lens,  the  same  as  with  the 
porte  lumiere. 

The  following  is  a  list  of  apparatus  and  of  experi- 
ments which  are  suitable  for  such  projection :  Equil- 
ibrium of  the  same  liquid  in  several  communicating 
vessels;  equilibrium  of  different  liquids  in  communi- 
cating vessels  ;  cartesian  diver  ;  the  hydrometer  j  cap- 
illarity ;  diffusion  of  gases ;  Torricelli*s  experiment  j 
Mariotte's  law ;  the  manometer  ;  SprengePs  air  pump ; 
fountain  in  vacuo  ;  the  siphon ;  the  pyrometer ;  the  in- 
fluence of  pressure  upon  the  boiling  point ;  M.  Des- 
pretz's  experiment  on  the  conductivity  of  solids  ;  con- 
vection j  the  thermo-pile ;  umbra  and  penumbra ;  action 
of  magnets ;  attraction  and  repulsion  from  electrical  ex- 
citation. Natural  history  specimens,  such  as  birds,  rats, 
mice,  squirrels,  frogs,  toads,  live  fishes,  if  in  a  tank  with 
transparent  sides  ;  leaves  of  trees,  ferns,  etc. ;  well-de- 
fined crystals,  such  as  quartz,  feldspar,  mica,  pyrite  \ 
diagrams  on  glass  of  machinery,  as  the  steam  ^ngine^ 
these  diagrams  can  be  drawn  a  foot  square  or  more  \ 
silhouettes,  etc.,  etc.,  are  all  available  with  this  method. 

There  is  an  advantage  in  this  plan,  when  it  is  at  all 
applicable,  that  will  commend  itself  to  every  one, 
namely,  it  is  available  at  any  point  between  the  focus 
and  the  screen,  hence  it  will  only  be  necessary  to  place 
the  object  in  the  path  of  the  rays  to  the  screen  at  such 
a  point  as  will  be  convenient  and  will  make  the  shadow 
sufficiently  large.  The  instructor  can  stand  by  the  ob- 
ject, and  with  a  pointer  like  a  pencil  call  attention  to 
any  particular  part.  And  again,  the  field  is  so  large 
that  several  objects  can  be  in  it  at  a  time,  if  need  be, 
for  comparison,  such  for  instance  as  leaves  of  several 
species  of  oaks  or  maples,  or  a  range  of  capillary  tubes 
of  various  diameters. 


38  THE  ART  OF  PROJECTING. 

THE    MEGASCOPE. 

Photographs  that  are  taken  especially  for  projection 
with  the  magic  lantern  are  often  called  transparencies 
because  all  of  the  lighter  parts  of  the  pictures  are  made 
as  transparent  as  possible,  and  they  are  shown  by  light 
that  is  transmitted  through  them.  If  one  would  ex- 
hibit a  picture  like  a  stereoscopic  view  or  a  common 
carte  de  visite,  it  is  evident  that  recourse  must  be  had  to 
some  other  arrangement.  The  light  must  be  reflected 
from  the  picture,  but  when  only  the  ordinary  amount 
which  is  reflected  from  a  surface  of  nine  square  inches 
is  distributed  over  seventy-five  or  a  hundred  square  feet, 
it  is  evident  that  it  will  be  but  dimly  visible.  If  a  large 
amount  of  light  is  concentrated  upon  the  picture  it  will, 
of  course,  reflect  more,  and  its  image  will  be  corres- 
pondingly brighter.  This  can  be  effected  in  two  ways : 
first,  by  using  a  large  lens,  or  second,  by  using  a  large 
concave  mirror. 

The  following  figures  will  serve  to  show  how  this  may 


Fig.  22, 

be  done.  When  sunlight  is  used,  the  larger  the  con- 
denser the  better.  One  seven  or  eight  inches  in  diam- 
eter, if  possible,  should  concentrate  the  light  upon  a 


PROJECTIONS.  39 

second  plain  mirror  at  r,  which  should  have  such  an 
inclination  as  to  reflect  the  converging  rays  upon  the 
object  to  be  shown  at  d,  and  strongly  illuminate  it ;  the 
objective  at  o  will  be  used  in  the  same  way  as  for  any- 
other  projection.  This  apparatus  should  be  in  a  box 
made  with  sides  a  foot  square  and  six  or  eight  inches 
deep.  At  the  back  of  it  a  hole  should  be  left  at  d,  in 
which  the  various  objects  for  exhibition  may  be  held. 

In  place  of  the  condenser  and  the  plain  mirror,  a 
large  concave  reflector,  such  as  is  used  behind  lamps, 
may  be  placed  at  r,  and  the  parallel  rays  from  the  porte 
lumiere  allowed  to  fall  upon  it.  It  should  be  placed  at 
such  a  distance  from  the  object  d^  that  it  will  just  illu- 
minate it  j  this  will  of  course  be  determined  by  the 
focal  length  of  the  mirror. 

The  room  needs  to  be  quite  dark  for  the  successful 
working  of  this  apparatus,  and  especial  care  should  be 
taken  to  prevent  any  of  the  light  from  th&  porte  lumiere 
from  being  scattered  into  the  room  ;  paint  the  box  black, 
inside  and  out,  with  lampblack  mixed  in  japan  varnish. 

If  the  lime  light  be  used,  as  it  generally  is  for  such 
an  exhibition,  it  is  necessary  to  modify  the  lantern  very 
much, — so  much  so  as  to  require  an  entirely  new  in- 
strument. The  following  is  the  simplest  plan  of  one : 
A  square  wooden  box  made  eighteen  or  twenty  inches 
on  a  side,  and  about  fifteen  inches  deep,  may  have  a 
little  way  made  in  it  on  one  side  for  the  fixtures  holding 
the  jet  i  and  the  lime  /  to  slide  upon.  A  hole  r  cut 
six  inches  square,  may  be  made  near  the  corner,  and 
another  one  on  the  front  side  for  the  light  to  come 
through  upon  the  lens  o,  which  is  the  only  lens  needed 
for  work.  The  size  of  this  hole  should  be  no  greater 
than  that  of  the  lens  o  used  for  the  projections,  but  this 
lens  should  be  as  large  as  possible.    A  lens  six  or  eight 


40 


THE  ART  OF  PROJECTING. 


inches  in  diameter,  with  a  focus  of  from  eighteen  to 
twenty-four  inches,  will  be  found  best  for  the  purpose. 

This  may  be  held  in  the  re- 
tort-holder before  mentioned, 
and  set  at  such  a  distance  in 
front  of  the  hole  that  an  ob- 
ject c,  when  strongly  lighted, 
will  be  plainly  projected  upon 
the  screen  s.  The  whole  of 
the  back  on  the  in  side  should 
be  covered  with  white  jjaper. 
Let  a  black  cloth  flap  hang 
over  the  hole  at  r,  so  that  no 
light  will  enter  the  room,  save 
what  is  reflected  from  the  il- 
luminated object. 

With  these  conditions  a 
dark  photograph  of  an  in- 
dividual, upon  a  white  background,  will  show  quite 
well.  Objects  held  in  the  hand,  such  as  a  watch  with 
its  movements,  cameo  pins,  small  flowers,  surface  of 
half  an  apple  or  orange.  The  latter,  if  squeezed  when 
being  shown,  presents  a  very  amusing  appearance. 
Minerals,  crystals,  shells,  bright-colored  beetles,  bugs, 
butterflies,  etc.,  may  all  be  exhibited,  and  appear,  with 
the  shades  and  shadows,  like  real  objects.  This  con- 
stitutes the  megascope. 

The  accompanying  cut  ( Fig.  24 )  represents  the 
scenic  effect  of  the  human  hand,  as  projected  by  the 
megascope. 


Tig.  23. 


THE   VERTICAL   ATTACHMENT. 

It  is  often  very  desirable  to  project  such  phenomena 
as  the  ripples  upon  the  surface  of   water,    the  move- 


PROJECTIONS. 


41 


Fig,  24. 


42  THE  ART  OF  PROJECTING. 

ments  of  a  horizontal  galvanometer  needle,  etc.,  such 
as  cannot  be  exhibited  with  the  common  forms  of  ap- 
paratus for  projections.  At  first  the  awkward  method 
was  adopted  of  turning  the  lantern  up  so  that  it  rested 
upon  its  back.  This  endangered  the  condensing  lenses 
of  the  lantern  from  the  great  heat  immediately 
under  them.  Dr. 
J.  P.  Cook  and  Dr. 
Morton  have  great- 
ly   improved    upon 


Fig.  25.  Fig.  26. 

this,  and  have   added  a  most  valuable   attachment  to 
the    lantern. 

The  cut  (Fig.  25)  represents  this  invention.  It  con- 
sists of  a  plane  mirror  inclined  at  an  angle  of  45°,  and 
when  so  placed  that  the  beam  of  light  from  the  lantern 
falls  upon  it,  it  is  reflected  perpendicularly  upwards 
upon   a  lens  that  converges  the   light  when  it  passes 


PROJECTIONS. 


43 


through  the  objective  above  it,  and  falls  upon  a 
second  mirror,  which  is  so  mounted  as  to  allow 
reflection  in  any  direction.  The  same  device  is 
made  a  part  of  the  "  College  Lantern^''  manufactured 
by  Hawkridge,  of  Hoboken,  N.  J.  By  an  ingenious 
arrangement  the  change  from  the  horizontal  to 
the  vertical  can  be  made  in  less  than  half  a  minute. 
The  microscope,  the  polariscope,  the  electric-light 
regulator,  and  several  other  fixtures,  are  fitted  to 
this  instrument,  making  it  a  most  perfect  and  complete 
lantern. 

Such  a  vertical  attachment  as  is  shown  in  Fig.  25  is 
applicable  to  the  porte  lumiere,  but  one  can  be  extem- 
porized, that  will  do  good  service,  with  such  material  as 
is  accessible  to  every  one.     An  iron  filter-stand,  such 


Fig,  27. 

as  is  in  common  use  in  every  chemical  laboratory,  may 
be  taken,  and  the  condensing  lens  c  laid  upon  the  lower 
or  largest  ring,  and  the  objective,  0,  upon  the  upper  or 
smaller  one,  as  shown  in  Fig.  27.  Below  the  lower  ring. 
a  plain  mirror  m  may  be  placed,  at  such  an  inclination 
that  the  beam  of  parallel  rays  falling  upon  it  from   the 


44  THE  ART  OF  PROJECTING. 

porte  lumiere  will  be  reflected  upward  through  the  two 
lenses  upon  another  smaller  mirror,  n^  which  may  be 
held  in  a  retort-stand,  and  the  beam  directed  to  the 
proper  place. 


PHYSICAL  EXPERIMENTS. 


DIVISIBILITY    OF    MATTER. 

A  good  way  to  show  the  minute  divisibility  of  matter 
is  to  dissolve,  in  water,  a  small  quantity,  say  a  gram,  of 
cupric  sulphate,  and  add  enough  ammonia-water  to 
make  a  clear,  blue  solution.  Put  it  into  the  chemical 
tank,  having  measured  its  capacity  in  cubic  centimeters, 
or  inches,  fill  it  with  water,  and  project  the  tank  by  the 
method  described  on  page  183.  A  beautiful  blue  color 
will  appear  upon  the  screen.  With  a  small  syphon  of 
bent-glass  tube,  draw  out  one-half  of  the  solution  and 
fill  up  with  pure  water.  The  amount  of  coloring  mat- 
ter will  be  reduced  one-half,  but  the  solution  will  be 
strongly  colored.  Remove,  in  the  same  way,  another 
half,  and  so  on  until  the  blue  color  is  no  longer  visible 
— comparing  the  color  with  that  of  pure  water,  pro- 
jected, at  the  same  time,  in  a  test-tube.  Keep  account 
of  the  number  of  dilutions,  and  at  last,  when  the  blue 
color  is  on  the  vanishing  point,  calculate  the  weight  of 
cupric  sulphate  in  each  cubic  centimeter  of  water.  In 
place  of  the  copper  solution,  any  of  the  analine  dyes 
will  do  as  well. 

The  same  thing  can  be  illustrated  with  a  soap-bubble, 


PHYSICAL  EXPFKIMENTS.  45 

blown  thin,  and  projected  in  the  diverging  beam  (Fig. 
2i).  The  bubble  will  be  sharply  defined  upon  the 
screen,  and  its  magnitude  will  depend  upon  the  diverg- 
ence of  the  beam  of  light,  and  its  distance  from  the 
screen.  It  may  be  made  ten  or  fifteen  feet  in  diameter, 
if  the  lens  have  a  short  focus.  The  colors  will  begin  to 
appear  around  the  pipe  in  bands,  and  computation  of 
the  thickness  may  be  made,  and  of  the  probable  num- 
ber of  molecules  in  its  thickness.  For  the  considera- 
tion of  this,  see  "  The  New  Chemistry,"  by  Professor 
Cooke,  and  Nature,  Vol.  I,  p.  551;  also  Galloway's 
"First  Steps  in  Chemistry,"  article  102. 

POROSITY. 

The  gases  dissolved  in  common  water  will  be  ex- 
pelled by  gently  heating  some  in  a  test-tube  while  the 
whole  is  projected.  The  bubbles  will  be  seen  to  form 
and  rise  where  nothing  was  before  visible.  The  po- 
rosity of  water  can  be  shown  by  projecting  a  test-tube 
half  filled  with  it,  and  its  depth  marked  by  a  bit  of 
thread  tied  about  the  tube  at  the  level  of  the  surface. 
A  considerable  quantity  of  salt  or  sugar  can  be  added 
to  the  water  without  noticeably  increasing  its  bulk.  A 
piece  of  chalk  dropped  into  a  test-tube  containing 
warm  water  will  at  once  give  out  quite  a  quantity  of 
included  air. 

The  ordinary  experiment  of  showing  the  porosity  of 
leather  by  forcing  mercury  through  it  by  atmospheric 
pressure  into  a  partial  vacuum,  can  be  exhibited  by  pro- 
jecting the  upper  part  of  the  lube,  while  tiie  exhaus- 
tion is  going  on.  The  mercury  will  be  seen  to  fall 
upward  on  account  of  the  inverting  by  the  lens. 

A  mixture  of  equal  parts  of  strong  sulphuric  acid 
and  water  loses  notably  in  volume  when  cool.     Fill  a 


46  THE  ART  OF  PR OJE C TING. 

test-tube  with  the  fresh  mixture,  and  tie  a  string  about 
the  tube  at  the  hight  of  the  mixture.  It  will  be  too 
hot  for  handling  with  the  fingers  at  first,  but  it  may  be 
cooled  in  a  few  minutes  enough  to  show  the  shrinkage, 
by  stirring  it  in  a  dish  of  cold  water.  The  surface  will 
be  seen  to  be  considerably  below  the  string  which 
marked  its  original  hight.  This  experiment  may  be 
used  to  exhibit  compressibility  of  liquids. 

Most  of  the  experiments  which  are  suitable  for  pro- 
jection of  the  properties  of  matter  are*  chemical,  and 
will  be  found  described  under  that  head.  Diagrams, 
such  as  are  given  in  most  text-books  on  mechanics,  can 
be  made  upon  glass  by  one  of  the  processes  described 
on  page  30,  and  will  be  found  very  convenient  to  a  lec- 
turer upon  that  subject. 

COHESION. 

A  drop  of  water  or  other  fluid  exhibits  this,  and  may 
be  projected  with  the  lantern,  or  with  the  porte  lumiere, 
and  a  single  lens  (Fig.   28).     Sprinkle  a   little  lamp- 
black or  lycopodium-powder 
upon  one  side  of  a  strip  of 
glass,  like  a  microscope  slide- 
and  place  it  in  the  proper 
place   for   projecting,  keep- 
ing  it   horizontal    that   the 
dust    may    not    slide     off. 
Fxg,  28.  '^o^  place  a  single  drop  of 

water  upon  the  slide ;  the  powder  will  prevent  it  from 
spreading  upon  the  glass,  and  it  will  gather  itself  up 
into  a  round  globule  with  some  of  the  dust  over  its  sur- 
face, making  an  interesting  object  upon  the  screen. 

Again,  a  saturated  solution  of  zinc-sulphate  is  put  into 
a  white   glass  square  bottle,   two   inches  square^   and 


PHYSICAL  EXPERIMENTS.  47 

three  or  four  inches  high.  Let  the  bottle  be  about  half 
filled  with  this  solution.  Into  a  few  drops  of  bisul- 
phide of  carbon  drop  a  piece  of  iodine.  It  will  at  once 
stain  the  bisulphide  a  dark-brown  color,  which  should 
then  be  carefully  dropped^  upon  the  solution  of  zinc, 
where  it  will  float.  If  now  pure  water  be  carefully- 
added,  so  as  to  rest  upon  the  solution  of  zinc,  the  bi- 
sulphide will  collect  into  an  oblate  spheroid,  having  the 
appearance  of  brown-colored  glass.  A  square  bottle 
will  enable  one  to  project  it  better,  as  a  round  bottle 
would  make  a  cylindrical  lens,  and  the  projection  would 
be  indistinct,  unless  the  vessel  was  quite  large. 

Nearly  fill  the  large  tank  (Fig.  20)  with  alcohol,  and 
project  the  tank  with  the  lantern,  or  with  the  single 
lens  and  porte  lumiere.  Now  drop  upon  the  alcohol, 
with  a  glass  rod,  or  other  convenient  thing,  any  of  the 
aniline  dyes.  As  soon  as  the  dye  touches  the  alcohol 
it  will  go  straight  down  for  a  short  distance,  then  it  will 
branch,  and  these  will  shortly  branch  again,  and  so  on 
to  the  bottom  of  the  tank,  when  there  will  be  a  large 
number  of  branches.  Upon  the  screen  the  appearance 
will  be  as  if  a  tree  were  growing ;  if  at  short  distances 
apart  in  the  tank  drops  of  different  colors  are  placed, 
the  branches  will  interlace  and  produce  a  fine  effect.  A 
tank  of  coal-oil,  in  which  is  dropped  a  little  colored 
fusil  oil,  is  said  to  produce  an  entirely  different  figure. 

But  it  is  with  the  vertical  attachment  that  the  most 
novel  and  interesting  phenomena,  due  to  cohesion,  may 
be  shown.  For  this  purpose  it  is  necessary  to  have  a 
horizontal  tank,  made  by  cementing  a  ring,  an  inch 
broad  and  four  or  five  inches  in  diameter,  upon  a  plate 
of  clear  glass.  The  ring  may  be  made  of  glass,  or 
wood,  or  zinc.  This  is  to  be  placed  upon  the  hori- 
zontal condenser,  and  half  filled  with  pure  water,  the 


48  THE  ART  OF  PROJECTING. 

surface  of  which  is  to  be  projected.  Let  fall,  from  a 
height  of  two  or  three  inches,  a  single  drop  of  ether. 
It  assumes  a  characteristic  form,  will  move  about,  but 
will  last  only  a  few  seconds,  as  it  evaporates  rapidly. 
Rinse  out  the  tank,  and  fill  again  with  pure  water,  and 
in  like  manner  drop  upon  its  surface  any  of  the  essen- 
tial oils,  of  creosote,  lavender,  turpentine,  sperm,  and 
colza  oils.  Each  one  will  assume  its  peculiar  form  due 
to  cohesion. 

Fig.  29  represents  the  pattern  exhibited  by  a  single 
drop  of  oil  of  coriander,  and  Fig.  30  the   appearance 


J /</.  29.  Fig.  30. 

of  oil  of  cinnamon.  Some  of  these  forms  are  very 
beautiful,  as,  for  instance,  that  due  to  oil  of  lavender. 
This  method  of  studying  oils  is  used,  by  some  experts, 
to  determine  their  kind  and  purity.  These  forms  are 
known  as  Tomlinson's  Cohesion  Figures. 

Again,  into  the  same  tank,  well  cleaned  and  filled  with 
water,  drop  a  few  small  pieces  of  camphor-gum.  As 
soon  as  they  touch  the  water  they  will  begin  to  move 
rapidly,  dodging  each  other  in  a  wonderful  way,  and  ap- 
pearing as  if  they  were  endowed  with  life.  Their  move- 
ments will  be  accelerated  if  the  water  is  warmed  to  a 
hundred  degrees,  or  a  little  more. 


PHYSICAL  EXPERIMENTS.  49 

A  drop  of  a  solution  of  camphor  in  sulphuric  acid, 
gently  delivered  to  the  surface  of  the  water,  will  take  a 
double-convex  lens  shape,  and  will  move  about  the 
water  in  an  eccentric  manner  for  a  long  time.  Several 
drops  may  be  placed  upon  the  water  at  a  time,  but  they 
will  avoid  each  other  in  their  movements.  Make  a 
small  boat  of  tin-foil,  and  into  it  put  a  fragment  of  cam- 
phor about  the  size  of  a  pea,  and  place  it  on  the  tank  j 
it  may  move  round  slowly,  but  put  a  piece  of  camphor, 
about  the  size  of  a  canary-seed,  upon  the  water,  and  it 
will  spin  round,  dart  up  to  the  boat,  and  drag  it  about 
in  a  lively  manner,  just  as  an  insect  might  do. 

To  show  the  existence  of  the  camphor-film,  that  forms 
upon  the  surface  as  soon  as  it  touches  it,  dust  the  sur- 
face of  the  water  with  lycopodium,  then  gently  lower  a 
fragment  of  camphor  upon  the  middle  of  the  tank. 
The  instant  the  camphor  touches  the  water  the  dust 
will  be  seen  to  open  out  into  a  circle  of  large  di- 
ameter j  then,  after  a  moment's  pause,  the  lycopodium 
is  formed  into  a  number  of  wheels,  arranged  in  pairs, 
revolving  in  opposite  directions. 

A  large  drop  of  camphor  dissolved  in  benzole, 
dropped  upon  water,  has  the  appearance  of  a  double 
convex  lens  ;  it  sails  slowly  about  for  a  while,  becoming 
flatter  and  thinner,  till  at  last  it  has  sudden  contrac- 
tions, assuming  different  shapes.  The  contractions 
multiply  till  at  length  they  become  so  violent  as  to  throw 
off  portions  of  the  disk,  or  split  up  into  smaller  disks, 
which,  in  their  turn,  twist  and  double  up,  and  ultimately 
throw  out  from  each  a  tiny  film  of  camphor,  which  lies 
quiet  upon  the  water. 

One  who  is  interested  to  pursue  this  subject  further 
will  find  an  abundance  of  material  by  Tomlinson,  in 
the  Philosophical  Magazine  for  1861.  Also  in  "  Experi- 
mental Essays,"  Weale's  Series,  No.  143. 


5o  THE  ART  OF  PROJECTING. 

CAPILLARITY. 

The  chemical  tank  (Fig.  20),  containing  a  little  colored 
water,  may  be  projected  in  any  convenient  way.  If  a 
small  glass  tube  be  placed  vertically  in  the  tank,  the 
solution  will  rise  in  it.  A  series  of  five  or  six  tubes, 
with  bores  of  different  size,  may  be  placed  in  this  tank 
at  the  same  time,  and  the  whole  projected.  The  water 
will  be  seen  to  rise  higher  as  the  tube  is  smaller.  A 
plate  of  glass  three  or  four  inches  square  may  be  put 
down  into  the  tank,  bringing  one  of  its  edges  against 
one  side  of  the  tank.  The  water  will  rise  two  or  three 
inches  where  the  glasses  touch,  and  slope  away  with  a 
beautiful  curve,  which  will  vary  as  the  whole  side  of  the 
glass  is  nearer,  or  more  distant  from  the  other  one. 

CRYSTALLIZATION. 

It  is  always  fascinating  to  watch  the  growth  of  crys- 
talline forms,  especially  when  the  process  can  be  leis- 
urely studied  over  a  surface  fifteen  or  twenty  feet 
square.  In  all  cases,  a  high  magnifying  power  will  be 
needed.  Three  hundred  or  four  hundred  diameters  is 
better  than  any  less. 

If  this  is  to  be  shown  by  a  lantern,  it  will  be  neces- 
sary to  have  a  powerful  light,  and  the  attachment  known 

as  the  microscope  at- 
tachment (Fig.  31), 
which  fits  upon  the 
lantern  (Fig.  26)  when 
adjusted  for  horizontal 
projection.  The  lens 
must  run  forward  nine 
or  ten  inches,  and  the 
*•''*       *  jet   drawn   back    until 

the  maximum  of  light  goes  through  the  objective,  which 


PHYSICAL  EXPERIMENTS.  $1 

has  a  short  focus,  and  will  not  be  more  than  three- 
fourths  of  an  inch  in  diameter.  A  strip  of  clear  glass 
an  inch  wide  and  three  or  four  inches  long,  will  answer 
upon  which  to  spread  the  solutions  to  be  examined,  a 
few  of  which  are  given  in  another  place.  The  glass 
will  then  only  need  to  be  placed  in  its  receptacle,  and 
its  front  focused,  the  same  as  for  any  microscopic 
objects.  Any  further  instructions  that  may  be  needed, 
may  be  found  under  the  descriptions  of  the  method 
with  the  solar  microscope.  With  the  porte  lumiere,  and 
two  lenses  of  proper  focal  length,  the  finest  effects  can 
be  shown. 


Fig.  3'4. 

Let  c  be  the  condenser,  with  say  twelve-inch  focus,  o 
the  objective  with  one-inch  focus ;  it  may  be  a  common 
pocket  lens,  or  a  linen  prover,  or  a  botanical  glass. 
First  adjust  c  so  as  to  give  a  disk  of  light  upon  the 
screen.  Tiie  rays  will  cross  at  the  focus,  and  diverge 
afterward.  Place  the  lens  o  so  that  all  the  light  may 
pass  through  it,  or  as  much  as  possible ;  this  will  de- 
pend upon  the  size  of  o.  At  any  rate,  it  will  be  near 
the  focus  of  c.  Have  ready  a  slip  of  glass  three  or 
four  inches  long  and  an  inch  wide,  and  wet  one  side 
with  the  solution  to  be  crystallized ;  as,  for  instance, 
ammonium  chloride,  sometimes  called  sal  ammoniac. 
Place  it  back  of  the  objective  at  g^  and  move  it  until 


52  THE  ART  OF  PKOJECTINU. 

the  wetted  surface  appears  very  plain  upon  the  screen. 
Then  wait  until  the  solution  begins  to  evaporate,  as  it 
will,  from  the  upper  edge  first,  when  crystallization  will 
begin  there.  See  to  it  that  the  focus  is  right,  and  then 
gently  blow  upon  the  plate,  unless  the  work  is  going  on 
fast  enough.  The  crystals  will  shoot  out  and  grow 
while  one  looks,  until  they  cover  the  entire  screen  with 
beautiful  forms. 

The  following  are  good  substances  for  illustration 
when  dissolved  in  water :  Ammonium  chloride  ;  barium 
chloride ;  copper  sulphate ;  camphor  dissolved  in  water  ; 
common  alum ;  urea  dissolved  in  alcohol. 

ICE    FLOWERS. 

To  exhibit  the  decrystallization  of  ice,  which  was 
first  shown  by  Tyndall,  it  will  be  necessar}^  to  saw  from 
a  very  clear  piece  of  ice  a  cake  three  or  four  inches 
square,  and  about  a  half  or  three-quarters  of  an  inch 
thick,  cut  parallel  to  the  plane  of  freezing.  When  first 
cut,  the  sawn  surface  will  be  too  rough  for  use,  but  will 
quickly  melt  smooth  enough  by  dipping  a  few  seconds 
in  water.     The  beam  of  light  that  falls  upon  it  should 


JFiflr.  33. 


consist  of  parallel  rays,  and  the  porte  lumiere  is  better 
for  projecting  this  experiment  than  any  lantern. 

A  single  lens  for  an  objective,  four  or  five   inchevS 


PHYSICAL  EXPERIMENTS.  53 

focus  or  longer,  will  answer.  It  is  the  interior  of  the 
ice  that  is  to  be  projected,  and  as  there  is  a  multitude 
of  planes  within  it,  each  one  being  slowly  decomposed, 
the  light  will  suffer  refraction,  and  one  must  not  look 
for  such  plain  figures  to  cover  the  screen  as  is  repre- 
sented in  Tyndall's  work,  -and  in  Deschanel's  Physics. 
The  forms  can  be  picked  out  here  and  there. 

If  a  lantern  be  used  to  project  these  crystalline  forms, 
remember  that  the  best  effect  will  be  obtained  with  a 
beam  of  parallel  rays,  which,  in  most  lanterns,  will 
necessitate  the  removal  of  the  front  lens  of  the  con- 
denser. 

THE   LEAD   TREE. 

Fill  the  small  glass  tank  for  the  solar  microscope 
with  a  rather  dilute  solution  of  the  acetate  of  lead  ;  ad- 
just it  as  for  the  exhibition  of  animalcules,  using  a 
small  lens  with  a  short  focus,  not  more  than  an  inch,  if 
such  an  one  is  possessed.  Into  the  solution  now  drop 
a  very  narrow  strip  of  sheet  zinc,  not  bigger  than  a 
common  sewing-needle;  such  a  piece  can  be  easily 
enough  cut  from  a  sheet  of  zinc  with  a  pair  of  shears. 
This  will  at  once  have  a  deposit  of  lead  upon  it  in  a 
beautiful  fern  structure,  which,  while  you  look  upon  it, 
grows  to  be  a  forest.  The  same  effect  can  be  produced 
in  the  larger  tank,  described  farther  on,  with  a  smaller 
magnifying  power,  by  using  a  small  battery  of  two 
Grove's  cells,  and  having  fine  platinum  wires  to  dip  into 
the  solution  of  lead.  The  lead  will  be  deposited  in  the 
fern-form  upon  one  of  the  wires.  After  there  is  a 
growth  of  the  crystals  upon  a  wire,  attach  the  other  end 
of  the  wire  to  the  other  pole  of  the  battery,  and  then, 
completing  the  circuit  again,  the  lead  will  be  dissolved 
from  the  first,  and  be  deposited  upon  the  second. 


54  THE  ART  OF  PROJECTING. 

THE    TIN    TREE. 

Take  a  rather  dilute  solution  of  chloride  of  tin,  made 
by  dissolving  the  crystalline  proto-chloride  in  water,  in 
the  proportion  of  one  part  of  the  former  to  four  or  five  of 
the  latter.  This  solution  will  precipitate  its  tin  upon  a 
piece  of  zinc  in  the  same  manner  as  the  lead  solution 
will,  but  the  form  of  the  crystals  is  very  different.  Use 
the  same  tank,  and  a  magnifying  power  of  400  or  500 
diameters,  if  good  sunlight  can  be  had.  The  growth 
will  be  quite  rapid,  and  crystals  six  or  eight  feet  long 
ought  to  appear.  This  needs  no  battery.  Solutions 
of  any  degree  of  concentration  can  be  used,  but  the 
growth  is  so  rapid  in  very  strong  solutions,  that  the 
masses  interfere  with  each  other,  and  are  dense  and 
imperfect  in  form.  Solutions  can  be  used  that  are  as 
dilute  as  twenty  or  more  parts  of  water  to  one  of  the 
crystalline  chloride. 

THE    SILVER    TREE. 

A  solution  of  nitrate  of  silver  is  put  into  the  tank, 
and  a  piece  of  fine  copper  wire  put  into  it,  the  wire 
being  nicely  focused  upon  the  screen.  Pure  silver  will 
be  immediately  deposited  in  arborescent  forms  upon 
the  wire,  but  the  forms  will  vary  with  the  strength  of 
the  solution.  The  more  diluted  it  is,  the  finer  will  be 
the  threads  of  silver. 

It  is  better  to  place 
the  metal  w  that  is  to 
have  the  deposit  upon 
it,  whether  of  copper  or 
zinc,  so  that  it  is  just 
below  the  surface  {/) 
of  the  solution,  for  the 
:E'ig,  34.  reason  that  when  it  is 


PHYSICAL   EXPERIMENTS. 


55 


projected  it  is  inverted,  and  as  the  arborescent  deposit 
hangs  upon  the  wire,  it  will  appear  upright  upon  the 
screen,  and  so  have  a  closer  semblance  to  a  rapid  veg- 
etable growth. 

A  neutral  solution  of  the  terchloride  of  gold  will  give 
a  characteristic  growth  upon  a  piece  of  zinc,  but  the 
solution  should  be  quite  weak. 

Salts  of  copper  will  give  nodular  forms  upon  zinc,  if 
very  dilute,  and  a  dense  fringe  of  black  copper,  if  the 
solution  be  very  strong,  sometimes  terminating  in  quite 
large  crystals. 

GRAVITATION. 

Make  a  frame  like  the  picture,  consisting  of  two  up- 
right posts,  about  one  foot  long  and  one  inch  square, 
grooved  like  flooring  on  one 
side  of  each.  Fix  these  into 
a  board  {a)  about  eight  inches 
by  twelve,  for  a  support. 
Fasten  a  strip  across  the  top 
10  hold  them  steady.  They 
should  stand  about  five  inches 
apart.  The  flange  should  be 
cut  away  from  the  right-hand 
standard,  from  the  top  down 
about  five  inches,  so  that  the 
weight  b,  which  has  a  tongue 
on  each  end,  can  be  put  into 
its  place  and  be  free  to  move 
up  and  down  between  the 
Fig,  35.  standards.     A  plate  of  glass 

{c)  of  proper  width  must  fit  in  the  top,  and  be  fastened 
by  a  button  {d\  or  otherwise,  and  held  firmly  in  place. 
Procure  a  pocket  tuning-fork,  either  an  A  or  a  C,  and 


56  THE  ART  OF  PROJECTING. 

solder  to  one  end  a  piece  of  small  copper  wire,  so  it 
will  project  from  the  side  about  one-eighth  of  an  inch. 
A  gimlet-screw  can  be  cut  into  the  other  end  of  the 
fork,  so  that  it  can  be  tightly  screwed  into  the  wooden 
weight  b,  if  small  gimlet- holes  are  made  before.  There 
should  be  a  number  of  these  holes  bored  into  b^  at  such 
a  distance  from  the  front  edge  that  when  the  fork  is  in 
one  of  them,  the  wire  upon  the  prong  will  come  against 
the  surface  of  the  glass  plate  when  the  weight  b  is 
raised. 

In  order  to  use  this  instrument,  it  is  necessary  to 
coat  the  front  side  of  the  glass  c  with  smoke,  photo- 
graphic varnish,  or  a  very  thin  coat  of  white  wax.  Fix  it 
in  its  place  in  the  frame,  and  then  raise  the  weight  b 
(which  ought  to  weigh  two  or  three  pounds)  until  the 
top  of  the  tuning-fork  is  above  the  glass.  Seize  the 
two  prongs  of  the  fork  with  the  thumb  and  forefinger, 
and  pinching  them  close  together,  suddenly  let  it  drop. 
The  wire  finger  will  trace  a  sinuous  line  upon  the  pre- 
pared surface,  caused  by  the  vibration  of  the  fork 
during  its  descent.  The  undulations  will  be  seen  to 
increase  in  length  as  they  approach  the  bottom ;  but  as 
each  one  was  made  in  the  same  time  with  every  other 
one,  it  is  obvious  that  the  velocity  increased  as  it  was 
falling.  In  order  to  show  this,  it  is  merely  necessary 
to  put  the  apparatus  near  the  condensing  lens,  and 
project  the  face  of  the  glass.  The  line  traced  by  the 
fork  will  be  seen  upon  the  screen.  It  will  now  be  well 
to  measure  the  lengths  of  the  undulations,  which  can 
be  very  well  done  by  having  a  scale  in  millimeters 
etched,  or  otherwise  fixed  upon  another  glass,  which 
can  be  put  just  in  front  of  the  first,  when  the  number  of 
divisions  of  the  scale  to  each  undulation  can  be  counted, 
and  the  result  stated  in  mathematical  terms. 


PHYSICAL  EXPERIMENTS.  57 

plateau's  experiment. 

With  the  vertical  attachment  and  a  tank,  made  five  or 
six  inches  deep  and  with  a  plane  glass  bottom,  this 
beautiful  experiment,  which  so  well  illustrates  cohesion 
and  centrifugal  force,  may  be  projected.     Fig.  36  shows 

the  proper  conditions. 
A  wire,  zc/,  is  made  to 
revolve  vertically  i  n 
the  tank,  by  means  of 
a  little  pulley  driven 
by  a  cord  about  a  larg- 
er one,  at  f^  the  whole 
so  made  as  to  rest  upon 
the  edge  of  the  tank, 
Fig,  36.  and  supported  by  ears, 

as  shown.  The  wire,  w,  should  have  a  thin  disk  of  tin 
fastened  to  it  at  s,  for  a  surface  of  adhesion.  Now  the 
solution  may  be  one  of  alcohol  and  water,  so  graduated 
that  its  specific  gravity  shall  just  equal  that  of  the  oil 
used,  which  can  only  be  done  by  trial  in  a  test-tube  ; 
or  it  may  be  a  solution  of  zinc  sulphate,  and  the  sphere 
may  be  made  of  bisulphide  of  carbon,  with  a  little  iodine 
dissolved  in  it,  which  will  make  it  black,  as  in  the  for- 
mer experiment  described  under  the  head  "Cohesion." 
Here,  also,  the  solution  of  zinc  sulphate  and  water  will 
need  to  be  of  the  same  density  as  the  bisulphide  of 
carbon,  which  will  be  best  found  by  trial.  This  fixture 
must  be  placed  upon  the  apparatus  for  vertical  projec- 
tion, and  the  focus  adjusted  for  the  sphere.  If  the 
above  fixture  for  producing  the  rotation  be  made  of 
stiff  wire,  it  will  not  interfere  much  with  the  distinct- 
ness of  the  projection.  A  full  account  of  this  experi- 
ment, and  of  all  the  conditions  to  be  observed,  will  be 
found  in  the  Smithsonian  Report  for  1865,  p.  207. 


S8  THE  ART  OF  PROJECTING. 

ACOUSTICS. 

THE  TUNING-FORK. 

The  vibrations  of  an  ordinary  tuning-fork  may  be  ex- 
hibited in  the  following  way.  Having  made  the  fork  to 
vibrate,  hold  it  at  a  in  the  divergent  beam  (Fig.  21), 
and  swing  it  in  its  plane  of  vibration  at  right  angles  to 
the  beam  of  light.  Its  shadow  will  present  a  curious, 
fan-like  appearance.  If  the  fork  is  polished  it  will  re- 
flect enough  light  to  exhibit  the  same  appearance  when 
looked  at  while  vibrating  and  swmging. 

Another  way  is  to  hang  light  pith  or  cork  balls  so 
they  just  touch  the  fork,  or  other  sounding  body,  and 
project  the  ball  in  any  convenient  way.  As  soon  as  the 
body  begins  to  vibrate  it  will  drive  the  ball  away  from 
it.  Two  forks  in  unison  may  be  used  in  this  way,  to 
show  sympathetic  vibration.  Hang  a  cork  ball  half  an 
inch  in  diameter  so  it  will  just  touch  the  side  of  one  of 
the  forks  near  the  end,  and  project  the  ball  and  fork. 
At  some  distance  set  the  other  fork  to  vibrating,  and 
put  it  upon  its  resonant  case,  or  place  the  stem  upon 
the  floor  or  some  resonant  surface.  The  ball  will  be  at 
once  thrown  off  from  the  first  fork,  showing  that  it  has 
been  set  vibrating. 

Professor  Mayer  has  described  a  number  of  interest- 
ing experiments  to  illustrate  the  change  of  wave-lengths 
by  the  motion  of  translation  of  the  sounding  body,  in 
the  American  journal  0/  Science,  April,  1872. 

THE   KALEIDOPHONE. 

To  the  end  of  a  piece  of  steel  wire,  two  or  three  feet 
long,  and  an  eighth  of  an  inch  in  diameter,  0  I  (Fig.  37), 


ACOUSTICS. 


59 


fasten  with  marine  glue,  or  sealing-wax,  a  small  bit  of 

mirror,  about 
the  fourth  of 
an  inch  square. 
The  wire  must 
be  held  tightly 
at  some  point, 
in  a  vice  upon 
a  table.  The 
Fig.  37.  light  from  the 

porte  lumiere  falls  upon  the  plane  mirror  w,  and  is 
thence  reflected  upon  the  small  mirror  on  the  end  of 
the  wire  at  /,  whence  it  is  reflected  to  the  screen.  If 
the  wire  be  now  carefully  plucked,  it  will  give  a  line  of 
light  upon  the  screen,  but  will  probably  soon  change 
into  an  ellipse  or  a  circle.  If  the  wire  be  struck  with 
a  small  billet  of  wood,  like  a  hammer-handle,  there  will 
be  heard  two  sounds,  the  fundamental  with  some  over- 
tone that  will  give  a  beautiful  compound  figure  upon  the 
screen,  some  circle  or  ellipse  made  up  of  small  undu- 
lations, which  will  vary  as  the  wire  is  struck  in  different 
places.  If  the  wire  be  made  fast  at  its  middle,  and  the 
other  end  of  it  be  plucked,  the  end  with  the  glass  will 
take  up  the  vibrations  at  once  —  a  case  of  sympathetic 
vibration.  If  it  is  not  fastened  in  the  middle  there  will 
be  little  or  no  movement  when  the  lower  end  is  struck. 
(See  Tyndall  on  Sound,  pp.  133,  135.) 


melde's  experiment. 


To  one  prong  of  a  small  pocket  tuning-fork  tie  a 
piece  of  silk  thread,  six  or  eight  inches  long,  and  to 
the  other  end  tie  a  pin-hook  and  hang  upon  it  a  small 
weight,  say  a  shirt-button.     Project  this  with  the  large" 


6o  THE  ART  OF  PROJECTING. 

lens,  as  represented  in  Fig.  2>'^.  First,  with  the  fork 
held  as  indicated,  make  it  to  vibrate.  The  string  will 
divide  up  into  segments,  all  of  which  can  be  plainly  seen 
and  counted.     Second,  turn  the  fork  so  that  it  vibrates 


Fig.  38. 

in  a  horizontal  plain.  The  number  of  segments  will 
be  doubled.  Third,  hang  another  button  upon  the  pin- 
hook,  so  that  the  weight  will  be  doubled.  Count  the 
segments  while  the  fork  vibrates,  both  perpendicularly 
and  horizontally.  In  this  way  some  of  the  laws  of 
vibrating  strings  can  be  demonstrated. 

Fasten  a  small  piece  of  wire  to  one  prong  of  the  tun- 
ing-fork, and  when  the  latter  is  vibrating  draw  it  quickly 
across  a  piece  of  smoked  glass.  The  undulating  line 
will  show  well  when  projected. 

THE    OPEIDOSCOPE. 

Take  a  tube,  of  any  kind,  that  is  five  or  six  inches 
long  and  an  inch  or  more  in  diameter,  tie  a  thin  rub- 
ber membrane  or  a  piece  of  tissue-paper  over  one  end, 
and  on  the  middle  of  the  membrane  glue  a  piece  of 
looking-glass  that  is  not  more  than  the  eighth  of  an 
inch   square.     The    light  from    the  J>orfe  lumiere  falls 


ACOUSTICS.  6l 

upon  a  mirror  a,  and  is  received  upon  the  bit  of  mirror 
upon  the  end  of  the  tube.  The  open  end  of  this  tube 
is  to  be  held  at  the  mouth  and  various  sounds  produced, 
varying  in  pitch  and  intensity.  The  vibrations  of  the 
membrane  will  move  the  mirror,  and  the  beam  of  light 


Fig,  39. 

reflected  from  it  upon  the  screen  will  describe  various 
beautiful  and  regular  curves,  depending  upon  the  man- 
agement of  the  voice.  It  will  be  easy  to  find  some 
pitch  and  intensity  which  will  give  a  straight  line :  then, 
while  the  sound  is  being  made,  if  the  outer  end  be 
swung  sidewise  at  right  angles  to  the  line,  an  undulat- 
ing line  will  appear,  in  every  way  like  those  produced 
by  the  vibrating  tuning-fork  described  on  another  page. 
If  there  are  prominent  over-tones  in  the  sound  they  will 
be  made  apparent  by  their  interference,  giving  a  trace 
just  like  the  traces  upon  a  smoked  glass  by  Scott's 
Phonautograph.  The  forms  are  regular  enough  for  a 
tone  of  a  given  pitch  and  intensity,  to  enable  one  to 
write  his  music  with  them  for  notes ;  and  if  a  tune  like 
"Auld  Lang  Syne  "  be  tooted  in  the  instrument,  the  ef- 
fect is  quite  amusing".  The  size  of  these  figures,  at  the 
distance  of  fifteen  or  twenty  feet,  may  be  six  or  eight 
feet  or  more. 


62  THE  ART  OF  PROJECTING. 

CHLADNl'S   EXPERIMENT. 

A  glass  plate  of  any  form,  if  fixed  by  a  clamp,  will 
give  out  a  musical  sound  when  a  violin  bow  is  drawn 
across  its  edge.  If  the  surface  of  the  glass  be  strewn 
with  sand,  the  latter  will  be  arranged  in  some  symmet- 
rical form.  The  glass  plate  may  be  prepared  as  for 
the  magnetic  phantom,  and  the  sand  fixed  after  its 
acoustical  arrangement,  and  afterwards  projected  as  an 
ordinary  transparency.  It  is  generally  best  to  exhibit 
this  phenomenon  during  the  process  of  arrangement, 
and  this  will  require  the  fixtures  for  vertical  projection. 
The  glass  to  be  sounded  is  to  be  made  fast,  and  so 
placed  that  as  much  as  is  possible  of  it  is  over  the  con- 
denser of  the  vertical  attachment ;  then  the  sand  sprin- 
kled upon  it,  and  the  focus  adjusted  for  the  upper 
surface. 

When  the  bow  is  drawn,  the  sand  is  seen  to  arrange 
itself  according  as  the  plate  gives  out  one  sound  or  an- 
other, which  depends  upon  the  part  of  the  plate  that 
is  bowed,  and  where  it  is  damped,  also  upon  its  form. 
It  is  well  to  have  round,  square,  triangular,  and  hex- 
agonal pieces,  eight  or  ten  inches  in  diameter. 

To  show  water-waves  upon  a  Chladni  plate,  Professor 
Morton  has  devised  the  following  way:  A  plate  of  glass 
about  a  foot  square  is  so  held  by  its  middle  that  one 
corner  covers  the  condenser  of  the  vertical  lantern.  To 
this  corner  is  cemented  a  thin  ring  of  soft  rubber,  of 
about  five  inches  in  diameter,  and  into  this  water  is 
poured  to  the  depth  of  one-tenth  of  an  inch.  Project 
the  surface  of  the  water  and  then  draw  the  bow  across 
the  edge  of  the  glass,  as  in  the  other  cases,  so  as  to 
produce  a  musical  sound.  The  water  within  the  rub- 
ber ring  is  thrown  into  a  system  of  large  waves,  which 


ACOUSTICS.  63 

form  a  shaded  net-work  of  singular  beauty.  Drawing 
the  bow  so  as  to  produce  notes  of  different  pitch,  the 
waves  will  be  large  or  small  as  the  notes  are  low  or 
high,  and  with  a  mixed  note  it  is  possible  to  get  two  or 
more  systems  superposed. 

If  a  common  tuning-fork  be  struck  and  then  have 
one  of  its  prongs  put  in  contact  with  the  surface  of  the 
water  in  this  tank,  a  beautiful  radiation  of  ripples  may 
be  seen,  resembling  somewhat  the  arrangement  of  iron 
filings  about  the  poles  of  a  magnet.  The  motion  of 
water  in  a  shallow  bell-glass  can  be  projected  by  letting 
the  parallel  beam  from  the  vertical  lantern  go  through 
it,  doing  away  with  the  condenser,  as  the  vessel  itself 
would  act  as  a  lens  if  water  were  in  it.  The  bow  may 
be  drawn  across  its  edge  when  it  will  give  out  a  musical 
sound,  the  water  will  be  thrown  into  ripples,  and  a  large 
objective  might  be  used  to  project  the  whole  surface. 
The  bell-glass  may  be  filled  with  ether  or  alcohol,  and 
then  sounded.  Some  of  the  liquid  assumes  the  sphe- 
roidal form,  and  these  are  driven  over  the  surface  to  the 
nodal  lines.     (See  Tyndall  on  Sound,) 

MANOMETRIC    FLAMES. 

The  flame  of  a  candle,  or  lamp,  or  gas-jet,  if  a  lumin- 
ous one,  can   be  projected  upon  a  screen  by  using  a 

concave  mirror 
(Fig.  40).  It 
will  be  invert- 
ed and  magni- 
fied. If  while 
the  flame  is 
projected  the 
mirror  be  tilted 
Fig,  40.  so  as  to  swing 


64  THE  ART  OF  PROJECTING. 

the  beam  horizontally,  the  flame  will  appear  drawn  out 
mto  a  band  of  light,  due  to  persistence  of  vision.  But 
if  the  flame  be  not  a  bright  one,  the  image  will  be  too 
dim  to  be  useful,  if  the  screen  is  ten  or  fifteen  feet 
away.  The  intermittent  character  of  the  singing  hy- 
drogen flame  can  be  shown  in  this  way,  but  it  is  much 
better  to  use  common  gas  in  place  of  hydrogen,  as  the 
flame  is  much  brighter.  The  flame  of  commcn  gas 
may  be  made  still  brighter  by  passing  it  through  ben- 
zole or  naptha,  or  tow  saturated  with  ether.  The  room 
must  be  quite  dark.  (See  Tyndall  on  Sound,  p.  223.) 
In  the  American  edition  of  Atkinson's  Ganot's  Physics 
is  pictured  Koenig's  apparatus  for  observing  manomet- 
ric  flames.  In  place  of  the  rotating  reflector  use  the 
concave  mirror,  as  above,  and  the  same  figures  will  ap- 
pear upon  the  screen. 

One  can  make  a  tolerable  substitute  for  that  apparatus, 
if  gas  be  not  obtainable,  by  fastening  over  the  mouth 
of  a  small  two-inch  funnel,  such  as  is  used  in  chemical 
laboratories,  a  piece  of  tissue-paper  or  thin  rubber.  A 
piece  of  rubber  tubing,  two  or  three  inches  long,  may 
be  drawn  over  the  stem  of  the  funnel,  and  the  other 
end  drawn  over  the  mouth  of  a  common  jeweler's  blow- 
pipe. A  sheet  of  pasteboard  may  now  be  rolled  so 
large  that  the  broad  end  of  the  funnel,  which  has  the 
tissue-paper  pasted  to  it,  may  fit  snugly  in  it.  The 
whole  fixture  may  now  be  supported  in  any  way,  by 
means  of  retort  stands.  A  gas- flame  from  a  small 
round  orifice,  or  a  common  candle  may  be  used  for  the 
flame ;  the  end  of  the  blow-pipe  is  to  be  inserted  in  the 
blaze,  with  the  opening  upward.  If  now,  either  a  com- 
mon mirror  be  used  to  give  angular  motion  to  the  re- 
flected beam,  or  the  concave  mirror  to  reflect  the  flame 
upon  the  screen,  while  a  sound  is  made  in  the  large 


ACOUSTICS.  65 

tube,  it  will  disturb  the  flame  so  much  as  to  give  a  dis- 
tinctly serrate  image  either  upon  the  screen  or  in  the 
plain  mirror.     The  annexed  figure  will  give   an  idea  of 


Fig.  41. 

the  arrangement  mentioned :  a  is  the  tube  for  produc- 
ing sounds,  in  b  is  the  funnel  with  tissue-paper  over  its 
mouth,  c  rubber  connection  to  the  blow-pipe  //,  which 
opens  upward  into  the  flame  from  the  candle  e. 

THE    ORGAN-PIPE. 

The  vibrations  of  the  air  reed  of  a  sounding  organ- 
pipe  may  be  shown,  by  having  a  small  pipe  made  of 
iron  gas-pipe  and  blown  by  illuminating  gas,  which  may 
be  lighted  ;  and  when  the  pipe  is  sounding  the  reed  will 
be  seen  to  swing  backward  and  forward  in  front  of  the 
emhonchure.  That  it  really  vibrates  may  be  seen  by  re- 
flecting the  light  from  a  mirror  upon  a  screen,  and  tilt- 
ing the  mirror,  as  is  done  in  showing  the  manometric 
flames. 

mach's  experiment. 

The  movement  of  the  air  within  a  sounding  organ- 
pipe  has  been  studied  optically  by  Mach,  a  German 
physicist.  His  method  was  to  stretch  a  membrane 
across  the  node  of  a  pipe  with  glass  sides,  and  in  the 
open  end  he  ran  a  fine  platinum  wire  to  the  membrane, 
and  thence  out  to  be  connected  with  a  galvanic  battery. 


66  THE  ART  OF  PROJECTING. 

A  sponge  dipped  in  strong  sulphuric  acid  was  drawn 
along  upon  the  stretched  wire,  the  acid  gathering  itself 
up  into  small  drops  at  regular  distances  apart.     When 


Fig.  42. 

a  current  of  electricity  of  sufficient  strength  was  sent 
through  the  wire  it  was  heated  red-hot,  and  the  acid  was 
vaporized  in  dense  fumes  that,  on  account  of  its  great 
density,  sunk  down  toward  the  bottom  of  the  tube, 
making  so  many  gaseous  strings  hanging  from  the  wire. 
These,  of  course,  were  subject  to  the  motions  of  the 
air  in  the  tube,  and  when  the  other  end  of  the  tube  was 
sounded  by  wind  from  a  bellows,  the  free  end  partook 
of  the  vibrations.  The  motions  were  then  observed 
through  a  revolving  stroboscopic  disk,  described  further 
on.  Not  only  the  swaying  of  these  gaseous  threads 
was  observed,  but  some  of  the  Lissajous's  curves  were 
seen. 

I  think  it  highly  probable  that  the  motions  of  the  air 
in  such  a  sounding-tube  can  be  shown  to  an  audience, 
by  having  the  tube  with  glass  sides  filled  with  dense 
smoke,  and  a  strong  beam  of  light  converged  in  it,  and 
having  the  stroboscopic  disk  so  placed  that  the  focus  of 
the  lens  would  be  in  the  holes,  and  so  permit  a  large 
amount  of  light  to  be  used.  Where  the  node  was 
formed  no  movement  would  be  visible  ;  but  by  giving 
the  disk  a  suitable  velocity,  at  any  other  place  than  the 
node,  the  vibration  might  be  shown  in  its  different 
phases. 


ACOUSTICS.  67 


LISSAJOUS'S  CURVES. 

The  optical  method  of  studying  vibrations  is  attract- 
ive to  old  and  young,  lo  students  of  science,  and  to 
musicians ;  but  the  apparatus  generally  used  is  so 
costly  that  not  many  can  afford  to  purchase  it.  The 
following  directions  will  enable  any  one  to  have  a  pair 
of  the  tuning-forks  made  at  the  nearest  blacksmith's 
shop,  that  will  be  found  even  more  satisfactory  for  pro- 
jections than  the  more  costly  ones. 

Choose  a  piece  of  steel  that  is  an  inch  broad,  one- 
fourth  of  an  inch  thick,  and  about  four  feet  and  a  half 
long.  Have  it  made  into  two  large  tuning-forks,  one  of 
them  to  be  about  fifteen  inches  long,  and  the  other 
twelve  inches.  Let  the  tines  be  two  inches  apart,  and 
the  flat  sides  should  face  each  other  on  each  fork.  A 
stem  may  be  now  welded  upon  the  bend  j  it  3(iould  be 
about  five-eighths  of  an  inch  in  diameter,  three  or  four 
inches  long,  and  made  of  round  steel.  When  one  of 
these  forks  is  struck  in  the  manner  of  common  tuning- 
forks,  it  will  be  seen  to  vibrate  through  quite  a  large 
arc,  and  will  continue  to  vibrate  perceptibly  to  the  eye, 
for  half  a  minute  or  more.  If,  while  the  fork  is  vibrating 
the  stem  be  held  upon  a  table  or  floor,  or  some  other 
resonator,  a  deep  sound  will  be  heard,  and  the  larger 
one  will  make  about  fifty  vibrations  per  second,  while 
the  shorter  one  will  probably  make  seventy  or  seventy- 
five  vibrations  per  second.  A  stand  will  be  needed  for 
each  of  these,  and  may  be  made  by  mortising  a  post 
three  inches  square,  and  three  or  four  inches  high,  into 
a  board  eighteen  inches  long  and  ten  inches  wide 
(Fig.  43).     This  post  should  have  an  inch-and-a-hal£ 


68  THE  ART  OF  PROJECTING, 

hole  bored  through 
it  lengthwise,  s  o 
that  a  smooth  stem 
may  freely  turn  in 
it.  This  stem 
must  have  a  large 
head  upon  it,  thro' 

which  is  bored    ^ 

■^*^*  ^^'  hole  to  receive  the 

stem  of  the  fork.  Set-screws  should  be  provided,  to 
fasten  the  stems  in  their  proper  places.  These  sup^ 
ports  might  be  made  of  cast-iron,  in  which  case  they 
would  not  need  to  be  nearly  so  large. 

Next  make  four  slides  of  iron,  an  inch  and  a  half  or 
two  inches  long,  and  bent  so  as  to 
slide  upon  the  fork  and  be  fixed  with 
a  set -screw  where  it  is  wanted. 
These  are  for  loading  the  forks  and 
making  them  vibrate  slower,  as  they 
are  nearer  the  ends. 

Lastly,  each  fork  will  need  a  small 
mirror  fastened  to  its  end.  The 
small,  round  pocket  mirrors,  about 
an  inch  in  diameter,  I  have  found  to  answer  well  j  but 
care  should  be  taken,  in  selecting  these  glasses,  to  get 
plain  mirrors.  Most  of  these  small  ones  are  on  poor 
glass,  and  will  spread  a  beam  of  light  over  a  large 
space.  These  mirrors  may  be  fastened  to  the  end  of 
the  fork  with  the  cement  known  as  marine  glue,  and 
will  adhere  strongly  enough  for  all  careful  work ;  but 
sometimes  these  are  fitted  with  a  screw  in  the  back,  and 
screwed  into  a  tapped  hole  in  the  end  of  the  fork. 

A  still  better  way  to  fasten  this  small  mirror,  is  to 
cement   to  its  back  a  piece  of  rubber  as  long  as  the 


mg.  44. 


ACOUSTICS,  69 

breadth  of  the  fork,  a  quarter  of  an  inch  thick,  and  half 
an  inch  broad,  this  to  be  cemented  to  the  end  of  the 
fork.  The  fork  will  not  vibrate  at  all  with  this  attach- 
ment at  first  j  but  if  a  thin  wedge  is  cut  out  from  each 
side  of  the  rubber,  until  it  moves  very  freely,  the  vibra- 
tions of  the  fork  will  not  be  much  interfered  with  ;  at 
the  same  time  the  amplitude  of  the  vibrations  will  be 
much  increased. 

When  the  mirror  is  fastened  to  each  fork,  it  will  be 
advisable  to  determine  their  pitch,  which  may  be  done 
by  comparing  them  with  a  properly-tuned  piano,  organ, 
or  another  tuning-fork  with  known  pitch. 

EXPERIMENTS    WITH   THE    FORKS. 

/  The  Sinuous  Line.  Cut  ofT  most  of  the  light  from 
the  lantern  or  J>orte  lumiere  with  a  diaphragm,  so  that 
the  beam  is  not  more  than  an  inch  in  diameter  and 
consists  of  parallel  rays.     Adjust  the  fork  so  that  it 


JPig,  45, 

will  vibrate  perpendicularly,  and  place  it  so  that  the 
beam  of  light  will  fall  upon  the  mirror  at  its  end. 
This  should  be  again  reflected  to  the  screen  by  a  mir- 
ror m  held  in  the  hands,  to  swing  the  beam  around  the 
room.  When  the  fork  is  made  to  vibrate  by  striking  it 
with  a  small  billet  of  wood,  if  the  mirror  m  is  held  still. 


70  THE  ART  OF  PROJECTING. 

a  band  of  light  will  appear  upon  the  screen,  three  to 
five  feet  long,  depending  upon  the  amplitude  of  vibra- 
tion and  the  distance  to  the  screen.  If  now  the  mirror 
tn  be  turned  so  as  to  swing  the  beam  at  right-angles  to 
the  band  of  light,  a  long  sinuous  line  of  light  will  be 


wwwww 


JPig.  46. 

spread  upon  the  wall.  It  may  be  seen  to  be  forty  or 
fifty  feet  long  if  the  mirror  be  moved  fast  enough.  At 
the  time  the  fork  is  struck  attention  may  be  called  to 
the  sound.  If  two  beams  of  light,  about  half  an  inch 
apart,  and  one  above  the  other,  be  made  to  fall  upon 
the  first  mirror  while  it  is  vibratinsr,  and  the  mirror  m 


ITig.  47. 

(Fig.  45)  be  moved  as  before,  two  undulating  lines  will 
appear,  one  above  the  other  (Fig.  47),  with  phases  ex- 
actly corresponding.     Let  the  two  beams  of  light  be 


r\  r\  r\  r\  r\  r 


mxmjj 


J^Hg.  48. 

brought  side-by-side  and  they  will  appear  to  have  op- 
posite phases  (Fig.  48),  and  will  show  as  beautiful  in- 
terlacing lines.     A  double  image  prism  put  in  the  path 


ACOUSTICS.  71 

of  the  beam  just  in  front  of  the  fork,  serves  well  to 
give  this  double  line  of  light. 

//.  Overtone,  If  the  fork  be  struck  about  midway  of 
its  length,  a  much  higher  sound  will  be  heard  along 
with  the  fundamental.  Let  the  mirror  be  turning  when 
the  fork  is  struck,  and  the  large  sinuous  line  seen  be- 
fore will  now  be  seen  covered  with  ripples  due  to  the 
overtone. 

///.  Interference.  In  the  place  of  the  mirror  at  m, 
place  the  second  fork  so  that  the  beam  of  light  from 
the  first  will  fall  upon  it,  and  be  reflected  to  the  middle 
of  the  screen,  having  both  forks  to  vibrate  perpendicu- 
larly. Now  load  the  shorter  fork  with  slides  until  it  is 
nearly  in  unison  with  the  long  fork.  When  they  are 
both  made  to  vibrate,  the  line  of  light  upon  the  screen 
will  be  seen  to  lengthen  and  shorten  with  regularity ; 
at  the  same  time  beats  will  be  heard  corresponding  with 
the  lengthening  of  the  line.  These  beats  may  be  made 
to  vary  in  frequency  by  moving  the  slides.  If  the  beats 
are  as  many  as  five  or  six  a  second,  or  more,  and  the 
second  fork  be  swung  upon  its  vertical  axis,  the  inter- 


Pig.  49. 

ference  may  be  noted  (Fig.  49) ;  the  swellings  corres- 
ponding to  the  periods  of  coincidence,  and  the  con- 
traction to  the  periods  of  interference. 

If  the  forks  are  now  brought  to  unison  and  struck, 
the  resultant  figure  will  depend  upon  their  relative 
phases.  If  they  have  like  phases,  so  that  each  one 
reaches  its  limit  at  the  same  instant,  the  line  of  light 
upon  the  screen  will  be  much  elongated,  the  amplitude 


73  THE  ART  OF  PROJECTING. 

being  equal  to  the  sum  of  the  two  amplitudes.  If  their 
phases  are  opposite,  so  that  one  reaches  its  upper  limit 
at  the  same  instant  that  the  other  reaches  its  lower 
limit,  then  the  spot  of  light  will  not  be  drawn  out  into  a 
line  at  all,  but  will  remain  quiescent.  These  various 
relative  vibrations  can  only  be  obtained  by  trial,  but 
usually  in  four  or  five  strokes  one  will  develop  such  a 
phase  as  he  requires. 

IV.  Resultants.  Keeping  the  two  forks  in  unison, 
turn  the  second  fork  so  that  it  vibrates  horizontally. 
Adjust  the  light  so  that  it  falls  upon  the  second  mirror 
as  before,  and  thence  to  the  middle  of  the  screen. 
Now,  if  both  forks  be  struck,  the  resulting  figure  may 
be  a  straight   line,  an   ellipse,  or  a  circle  depending 


upon  the  phase  of  the  first  fork  when  the  second  one 
begins  to  vibrate.  Fig.  50  represents  these  unison 
forms.  By  moving  one  of  the  slides  so  that  the  fork  is 
not  quite  in  tune  with  the  other,  the  figure  will  move 
through  each  of  these  phases  alternately.     When  the 


Wig.  51. 

circle  is  obtained  upon  the  screen,  swing  the  second 
fork  through  a  small  arc,  and  the  circle  will  be  drawn 
out  into  a  luminous  scroll,  (Fig.  51).     If  the  forks  are 


ACOUSTICS.  73 

not  quite  in  unison,  the  same  experiment  will  give  the 
scroll  of  irregular  amplitude,  (Fig,  52). 


Fig.  52. 

Remove  the  slides  from  the  short  fork  and  fix  them 
upon  the  long  one  near  the  end,  and,  if  necessary,  at- 
tach two  pairs,  and  adjust  them  so  that  the  ratio  of  vi- 
brations is  as  2  to  I  ;  that  is,  their  pitch  is  an  octave 
apart.    The  resulting  figures  are  shown  in  Fig.  53  j  and 


Fig.  53. 

when  the  forks  are  tuned  exactly,  the  figure  first  de- 
veloped will  remain,  with  no  other  alteration  than  a 
decrease  in  size,  and  may  be  a  parabola,  an  8,  called 
a  lemniscata,  or  an  intermediate  form. 

While  this  figure  8  is  upojj  the  screen  let  the  second 


F%y.  34. 

fork  be  rotated  through  a  small  arc,  as  before  with  the 
unison,  and  the  scroll  shown  in  Fig.  54  will  appear. 

By  trial  the  slides  may  be  so  adjusted  upon  one  of 
the  forks  that  all  the  varying  ratios  in  the  octave  may 
be  obtained.     The  simpler  the  ratio  the  simpler  the 


74 


THE  ART  OF  PROJECTING. 


Fig.  55, 

figure,  and  such  ratios  as  2  to  3  {do  to  sol^^  and  3  to  4 


iij^urf,  cinu  &UU11  1  alius  as  z  lu  3  \uu  lu  jt/^y,  aiiu   3    lu  4 
{do  iofa)^  may  be  known  by  their  representative  figures, 


Fig,  56, 

55  and  56.     The  ratio  i  to  3  {do  to  sol^  in  next  octave,) 
will  present  such  forms  as  those  in  Fig.  57. 


Fig,  57, 

In  any  case,  the  figure  will  remain  constant  when  the 
ratio  is  exact,  and  the  ratio  may  be  known  by  counting 
the  number  of  loops  upon  the  top  and  one  side.  Thus, 
in  the  fully  developed  figure,  with  the  ratio  2  to  3,  there 
may  be  counted  two  loops  upon  the  top  and  three  loops 
upon  the  side,  which  indicate  that  the  fork  that  vibrates 
horizontally  makes  three  vibrations,  while  the  other  one 
makes  two. 

The  overtones  may  be  developed  and  exhibited  upon 
each  of  these  compound  forms  by  striking  upon  the 
fork  rather  lightly,  about  midway  of  its  length,  while  it 
is  giving  any  particular  figure.  Thus,  if  the  forks  are 
in  unison  and  a  circle  has  been  obtained,  the  overtone 


ACOUSTICS,  yr 

developed  will  cover  the  circle  with  ripples  which  ap- 
pear to  move  around  it. 

For  the  exhibition  of  the  Lissajous  curves  with  such 
forks  as  have  been  described,  it  is  not  necessary  to  use 
a  lens,  but  the  whole  light  from  the  porte  lumiere  may 
be  allowed  to  enter  the  room,  and  the  first  fork  placed 
with  its  mirror  in  the  middle  of  the  beam.  If,  however, 
it  be  desirable  to  admit  less  light  into  the  room,  a  dia- 
phragm may  be  used  that  admits  a  beam  only  an  inch 
in  diameter  or  less.  A  lens  may  be  used  which  will 
concentrate  the  light  upon  a  much. smaller, space,  mak-. 
ing  a  much  brighter  spot,  but  will  very  much  reduce  the 
size  of  the  figures.  When  a  lens  is  used,  it  must  be  so 
placed  as  to  project  the  mirror  upon  the  second  fork. 
Its  focal  length  should  be  two  feet  or  more. 

All  of  these  phenomena  can  be  shown  by  means  of 
a  lantern, — even  an  oil  lantern  will  answer.  It  will  be 
found  best  to  use  a  beam  of  parallel  rays,  which  may 
be  used  in  such  a  lantern  as  is  represented  in  Fig.  26 
by  simply  removing  the  front  lens  of  the  condenser. 
With  other  lanterns  it  will  be  necessary  to  remove  the 
objective,  and  push  forward  the  light  until  the  beam 
emerges  with  parallel  rays :  then,  with  a  diaphragm  cut 
off  all  the  light  except  a  beam  of  the  size  of  the  mir- 
ror upon  the  forks.  The  conditions  are  then  the  same 
as  with  sunlight,  and  a  lens  may  or  may  not  be  used. 

SYMPATHETIC  VIBRATIONS. 

Let  the  two  forks  be  brought  to  unison  and  at  right 
angles,  so  as  to  give,  when  struck,  one  of  the  forms  of 
Fig,  50.  If  now,  but  one  of  the  forks  be  struck,  the 
straight  line  due  to  its  vibration  will  slowly  swell  into 
an  ellipse,  which  will  be  due  to  the  absorption  by  the 
second  fork  of  the  vibrations  of  the- first.     This^ioay. 


76  THE  ART  OF  PROJECTING. 

be  demonstrated  by  changing  the  pitch  of  one  of  the 
forks,  when  no  change  of  form  of  the  projected  beam 
will  be  observed.  One  of  the  conditions  for  the  suc- 
cess of  this  experiment  is  that  both  forks  should  rest 
upon  the  same  table,  in  order  that  the  vibrations  may 
be  conveyed  through  the  solid  wood  from  one  fork  to 
the  other.  The  intensity  of  the  sound-wave  in  the  air 
is  not  sufficient  to  communicate  a  motion  that  will  be 
perceptible.  A  voice  sounding  the  same  fundamental 
note  as  one  of  the  forks,  will  set  it  vibrating,  as  will  be 
evident  by  the  spot  of  light  upon  the  screen  being 
drawn  out  into  a  line. 

With  one  of  these  forks  Melde's  experiment  may  be 
shown  in  the  most  satisfactory  manner.  Choose  a  soft 
white  cord  eight  or  ten  feet  long  (a  silk  cord  is  best,  though 
a  cotton  twine  will  work  very  well),  tie  one  end  to  the 
fork  at  a  and  let  the  other  end  hang  over  a  hook  driven 
in  the  wall  at  b.     Weights  varying  from  a  pound  to 


Fig.  5S. 
half  an  ounce  or  less  may  be  hung  upon  this  free  end 
of  the  string,  with  which  its  tension  may  be  varied. 
The  fork  may  be  struck  with  a  billet  of  wood,  as  in  the 
former  experiments,  when  the  string  will  be  made  to 
vibrate,  either  as  a  whole,  or  in  equal  segments,  the 
number  of  which  will  be  inversely  proportional  to  the 
stretching  weight.  The  amplitude  of  these  vibrations 
of  the  string  will  be  considerable,  and  if  the  string  vi- 
brates as  a  whole  it  may  be  eight  or  ten  inches,  or  even 


ACOUSTICS.  77 

a  foot  j  and  when  the  number  of  segments  is  as  many  as 
sixteen  or  twenty,  they  can  all  be  seen  and  counted  by 
a  large  number  of  persons  at  a  time.  If  the  string 
a,  by  is  twice  as  long,  and  may  reach  back  to  «,  the 
free  end  may  be  held  in  the  left  hand  while  the  fork  is 
struck  with  the  right.  It  will  then  be  very  easy  to  vary 
the  tension  of  the  cord  while  it  is  vibrating,  and  the 
segments  can  be  made  to  change  through  its  whole 
series  of  one,  two,  three,  four,  etc.  The  various  forms 
and  motions  of  the  cord  may  be  shown  to  still  better 
advantage,  by  making  a  strong  beam  of  light  from  the 
porte  lumiere  or  lantern  to  fall  upon  it  in  the  direction 
of  its  length. 

Crova's  apparatus  consists  of  disks  of  glass  about 
fifteen  inches  in  diameter,  which  may  be  made  to  turn 
upon  a  suitable  rotator.  These  disks  are  at  first  painted 
black,  and  then  curves  of  various  forms  are  traced 
through  the  paint  to  the  glass.  The  upper  part  of  the 
disk  is  projected  in  the  ordinary  way,  and  then  if  it  be 
rotated,  the  lines  which  are  drawn  upon  it  will  appear 
to  move  or  to  be  quiescent,  according  as  they  are  con- 
centric, eccentric,  or  some  other  form.  If  a  diaphragm 
with  a  slit  in  it,  long  enough  to  reach  across  all  the 
lines  which  are  drawn  upon  the  disk,  be  placed  behind 
it,  a  series  of  dots  will  appear  upon  the  screen,  which 
will  change  their  positions  as  the  disk  turns  round. 

With  properly  drawn  curves  the  various  wave-motions 
in  air  in  organ-pipes,  reflection  of  sound-waves,  nodes, 
interference,  and  so  forth,  as  well  as  the  transverse  vi- 
brations in  light-waves,  may  be  well  shown. 

AN    ATTACHMENT   TO    THE    WHIRLING    TABLE    FOR    PRO- 
JECTING  LISSAJOU'S  CURVES. 

Two  posts  p  and  p'  are  made  fast  to  the  frame  upon 
the   opposite  sides  of  the  inertia   plate   a.     A  small 


THE  ART  OF  PROSPECTING. 


wooden  pulley  s^  about  an  inch  in  diameter,  is  made  to 
turn  upon  an  axis  that  is  made  fast  in  the  post/,  and 
with  such   adjustment  that  the  pulley  rests  upon  the 


Fig,  59. 

plate  a  and  turns  by  friction  on  that  plate.  It  is  best 
to  have  a  thin  India  rubber  ring  upon  the  friction  pulley 
to  insure  it  from  slipping.  Above  the  pulley  the  mirror 
m  is  so  mounted  as  to  swing  in  azimuth,  and  is  made 
to  do  this  by  a  wire  fastened  to  it 
at  its  hinge,  and  bent  into  a  loop  2 
at  its  lower  end,  which  is  opposite 
the  face  of  the  pulley  s.  Another 
twist  in  the  wire  at  o  will  be  needed, 
for  a  pin  which  is  fast  in  the  post 
/ ;  this  will  make  a  lever  of  the 
wire  /,  with  the  fulcrum  at  o,  and  if 
it  is  properly  fastened  to  the  hinge 
Fig.  60.  Qf  ^jjg  mirror  will  cause  it  to  vibrate 

in  a  horizontal  plane  when  the  plate  a  revolves. 


ACOUSTICS.  79 

A  somewhat  similar  arrangement  :s  made  for  the 
other  side,  save  that  the  friction  pulley  sf  has  its  bear- 
ing made  fast  in  a  separate  piece  c,  which  is  so  fastened 
to  the  end  of  a  long  screw  d  that  the  whole  fixture  can 
be  moved  to  or  from  the  centre  of  the  plate  a.  The 
piece  c  is  furnished  with  two  guides,  which  keep  it 
steady  in  any  place  where  it  is  put.  The  mirror  m'  is 
made  to  tilt  in  a  perpendicular  plane  by  an  arrange- 
ment quite  similar  to  the  former  one,  save  that  the  wire 
connection  has  its  lower  end  bent  into  a  horizontal 
loop,  through  which  a  pin  in  the  face  of  the  pulley  /  is 
thrust.  This  is  practically  an  eccentric,  and,  being 
directly  fastened  to  the  hinge  of  the  mirror  /«',  gives  to 
it  an  angular  motion  proportional  to  the  distance  of  the 
pulley  face-pin  from  the  centre.  The  mirrors  should 
be  not  less  than  two  inches  square.  If  then  the  pin  is 
an  eight  of  an  inch  from  the  centre  of  the  friction  pul- 
leys, they  will  have  ample  angular  motion  j  much  larger 
than  can  ever  be  got  from  forks. 

Experiments. — It  is  evident  that  if  the  two  friction 
pulleys  have  equal  diameters,  and  they  are  at  equal  dis- 
tances from  the  centre  of  the  plate  a,  they  will  vibrate 
in  unison  in  their  respective  planes.  Now  let  a  beam 
of  light  r,  from  the  porte  lumiere,  fall  upon  the  mirror 
tn  at  such  an  angle  as  to  be  reflected  first  upon  the 
mirror  «?',  thence  to  the  screen.  If  the  plate  a  is  now 
revolved,  the  beam  of  light  will  describe  a  circle,  an 
ellipse  or  a  straight  line,  either  of  which  can  be  made 
at  will  by  simply  adjusting  the  crank  of  one  of  the 
mirrors  to  the  required  angle.  Thus,  suppose  the 
mirror  m'  is  tipped  back  its  farthest  by  bringing  the 
pulley  pin  at  the  top,  as  indicated  in  the  drawing,  at 
the  same  time  that  the  mirror  m  is  at  its  maximum  an 


8o  THE  ART  OF  PROJECTING. 

gular  deviation.  The  beam  of  light  will  describe  a 
circle. 

If  it  moves  slowly,  the  path  and  direction  of  the 
moving  beam  can  be  nicely  observed.  These  two  ad- 
vantages are  not  to  be  had  with  forks ;  for,  first,  it  is 
accidental  if  one  gets  a  circle  or  any  other  desired  re- 
sultant figures  from  forks  in  unison,  for  the  obvious 
reason  that  the  phases  cannot  be  regulated,  and  second, 
the  vibrations  of  the  fork  are  so  rapid  that  the  analysis 
of  the  motion  can  only  be  made  in  a  mechanico-mathe- 
matical  way. 

By  moving  the  fixtures  on  the  left  side  toward  the 
centre  of  the  plate  ^,  the  pulley  /  will  not  revolve  so 
fast.  If  moved  half-way  it  will  make  one  revolution 
while  the  other  makes  two,  and  the  vibrations  stand  in 
the  ratio  i :  2  represented  by  forks  in  octave.  Such 
ratio  is  shown  upon  the  screen  by  a  form  very  much 
like  the  figure  8,  and  known  as  the  lemniscate. 

Between  these  two  places,  every  musical  ratio  in  the 
octave  can  be  got,  and  the  resultant  motions  projected 
in  their  proper  curves.  More  than  that,  while  the  mir- 
rors are  both  vibrating,  any  of  the  ratios  desired  can  be 
moved  to  at  once  by  merely  turning  the  thumb  screw  d, 
which  is  wholly  impossible  with  any  forks  which  require 
stoppage  and  adjustment  of  lugs  for  each  different 
curve. 

Again,  if  the  fixture  c  is  moved  still  farther  toward 
the  centre  than  half-way,  the  curves  projected  will  be 
those  belonging  to  the  second  octave,  until  the  pulley 
reaches  three-fourths  of  the  way,  when  the  ratio  will  be 
1 :4,  and  the  resultant  figure  will  be  like  a  much-flat- 
tened double  eight. 

If  one  would  show  the  phenomenon  of  beats,  it  will 
be  necessary  to  have  the  mirror  m  and  its  attachment 


LIGHT.  81 

so  adjusted  as  to  have  it  vibrate  in  a  perpendicular 
plane  like  m' .  This  can  be  done  by  fixing  its  hinge  at 
right  angles,  and  the  rest  the  same  as  for  mirror  m' , 
The  reflected  beam  from  the  second  mirror  may  be 
received  upon  a  large  mijror  held  in  the  hands,  and 
thence  reflected  upon  the  wall  or  screen. 


LIGHT 


RECTALINEAR    MOVEMENT. 

That  light  moves  in  straight  lines  can  be  shown  by 
admitting  the  light  from  the  porte  lumiere  through  a 
small  hole.  It  goes  straight  across  the  room,  and  its 
course  can  be  tracked  through  the  room  by  the  dust 
particles,  or  a  little  smoke,  which  it  will  light  up.  Also, 
by  having  the  room  otherwise  quite  dark,  permit  the 
light  to  come  in  the  round  orifice,  half  an  inch  in  diam- 
eter, as  it  is  reflected  from  the  landscape  outside,  and 
not  reflected  from  the  mirror.  The  room  is  thus  a  large 
camera  obscura,  and  an  inverted  image  of  the  landscape 
will  be  seen  upon  the  walls,  or  upon  a  small  screen  held 
a  foot  or  two  from  the  orifice.  This  image  will  be  par- 
ticularly strong  if  the  ground  be  covered  with  snow,  as 
much  more  light  is  reflected  from  that  than  from  grass 
or  foliage.  If  persons  are  passing  their  forms  wiU  be 
seen,  and  appear  as  if  walking  head  downward. 

Parallel  rays  A  will  be  reflected  from  the  mirror  of 


82  THE  ART  OF  PROJECTING. 

t\\Q  portelumiere,  whiles  converging  b  and  diverging  c  rays 
will  be  obtained  by  interposing  a  convex  lens  of  any 
size  in  the  path  of  the  parallel  rays. 


Fig.  61. 

Transparent  substances,  like  glass,  some  crystals, 
gases,  and  water  permit  the  rays  a  to  go  through  them 
and  appear  upon  the  screen.  Translucent  substances, 
like  paper,  ground  glass,  milk,  allow  but  a  few  scattered 
rays  to  go  through  them,  and  a  diffused  light  appears 
on  the  screen.  Opaque  substances,  such  as  metals, 
thick  pieces  of  wood,  stones,  etc.,  stop  all  the  light, 
reflecting  some  and  absorbing  the  rest. 

INTENSITY    OF    ILLUMINATION. 

When  the  lens  is  interposed  in  the  path  of  the  beam 
the  light  appears  as  a  circular  disk  upon  the  screen, 
and  as  the  rays  cross  each  other  at  the  focus  f^  that 
point  may  be  considered  as  the  source  of  light.  Cut  a 
sheet  of  paper  or  a  board  j",  one  foot  square,  and  hold 
it  any  distance  from  the  focus,  say  two  feet.  Its  shadow 
upon  the  screen  will  be  bounded  by  ^,  ^,  which  may 
be  measured  in  square  feet.  Now  move  the  paper 
to  /,  twice  as  far  from  the  focus,  and  again  measure 
the  shadow  b^  </,  it  will  be  but  one -fourth  the  size 
of   the   other,    proving   that    at   s   the   paper  received 


LIGHT.  83 

four  times  as  much  light  as  it  did  at  /.  Hence  the  in- 
tensity of  light  varies  inversely  as  the  square  of  the 
distance.  Other  measures  with  other  distances  can  be 
made  for  confirmation  :   a  good  exercise  for  scholars. 


Fig.  62. 

When  a  lantern  must  be  used  in  place  of  sunlight,  it 
will  be  necessary  to  remove  the  objective  and  move  the 
light  backward  from  the  condenser  until  a  sharp  focus 
is  produced  in  front,  and  then  work  in  front  of  that ;  or 
still  better,  remove  both  condenser  and  objective,  the 
outlines  of  shadows  will  be  quite  well  defined  with  the 
electric  light,  and  with  the  lime  light,  but  not  with  any 
oil  light. 

REFLECTION. 

The  reflecting  power  of  various  surfaces  can  be 
shown  by  holding  them  in  the  path  of  the  beam  from 
the  reflector.  Common  mirrors,  plain  glass,  colored 
glass,  metals  polished  and  unpolished,  woods,  horn, 
polished  stones,  paper,  will  all  exhibit  difference  in  this 
property. 

Reflection  from  the  two  surfaces  of  glass  is  seen 
upon  the  screen  when  the  parallel  rays  from  the  first 
mirror  reach  it.  Then  will  always  be  seen  two  or 
three  indistinct  images  of  the  sun,  side  by  side.     When 


84  THJE  ART  OF  PROJECTING. 

the  sun  is  near  the  horizon,  so  that  the  porte  lumiert 
is  nearly  horizontal,  more  of  these  reflections  will  ap- 
pear, due  to  multiple  reflections  upon  the  surfaces  of 
the  mirror.  These  can  be  magnified  a  good  deal  in 
the  following:  way.     Place  the  lens  o  at  about  its  focal 


Fig.  61. 

length  distant  from  the  orifice,  and  then  hold  another 
plane  mirror  r  so  that  it  will  reflect  the  beam  upon  an- 
other screen  s,  moving  the  mirror  r  to  such  a  place  as 
to  project  the  image  of  the  orifice.  It  will  be  seen  to 
be  double,  and  when  the  images  overlap,  the  light  will 
be  much  brighter.  Multiple  reflections  from  the  two 
surfaces  of  the  mirror  r  may  be  seen  by  holding  it  at  a 
small  angle  to  the  beam  of  parallel  rays.  A  piece  of 
plate  glass  two  or  three  inches  square  answers  for  this 
experiment. 

That  the  reflected  beam  moves  through  twice  the 
angle  of  the  incident  beam,  may  be  shown  by  holding 
the  mirror  r  in  the  beam  without  the  lens  o.  If  the 
mirror  be  perpendicular  to  the  beam,  the  light  will  be 
reflected  back  through  the  aperture  ;  turning  the  mirror 
slowly  when  it  is  45°  to  the  incident  light,  the  beam 
will  be  overhead  90°  ;  when  it  has  been  turned  90°,  and 
is  now  in  the  plnne  of  the  beam,  the  reflected  part  will 
have  moved  through  180°. 


LIGHT. 


8S 


Pepper's  Ghost  is  but  a  reflection  from  the  surface  of 
unsilvered  glass.  His  fixtures  were  made  upon  a  large 
scale,  were  costly,  and  not  practicable  in  every  place. 
His  reflectors  were  large  sheets  of  glass  about  five  feet 


Tig.  69. 

broad  and  six  feet  high.  The  light  was  a  powerful 
lime  light.  Fig.  62  will  give  an  idea  of  the  conditions 
employed  last  year  in  his  traveling  lectures.  The  front 
of  the  stage  s  s  was  heavily  curtained,  except  a  space 
of  a  few  feet  in  the  middle  of  it,  where  there  was  a 
recess  opening  back,  and  apparently  to  the  back  of  the 
stage  r,  which  could  be  seen  through  a  large  plain  glass 
reflector  ^,  twelve  or  fifteen  feet  long  and  six  feet  high, 
placed  at  an  angle  of  about  45°.  This  glass  is  seldom 
noticed  unless  one  is  looking  for  it.  The  lantern  for 
illuminating  the  ghost  b  is  behind  the  curtain  on  the 


86  THE  ART  OF  PROJECTING. 

right,  and  throws  a  powerful  beam  upon  it.  It  being 
dressed  in  white,  a  good  deal  of  the  light  is  reflected 
from  it  in  all  directions,  and  a  part  of  that  which  falls 
upon  the  glass  at  r  will  be  again  reflected  toward  /, 
when  it  will  appear  as  if  it  came  from  r,  as  far  back  of 
r  as  ^  is  front  of  it.  All  of  the  lights  in  the  room  are 
turned  down  except  that  in  the  lantern,  and  none  of 
that  is  permitted  to  find  its  way  into  the  room  save 
what  is  reflected  from  the  ghost.  There  is  black  cloth 
for  absorbing  the  light  back  of  b.  The  person  who 
holds  conversation  with  the  phantom  is  at  d,  but  of 
course  he  cannot  see  what  those  see  who  are  at  /,  or 
near  that  line,  and  all  his  movements  are  guided  by  his 
knowledge  of  the  ne'cessary  position  of  the  reflection. 
In  his  book.  Cyclopaedic  Science  Simplified^  Professor 
Pepper  has  given  several  methods  for  showing  such 
spectra.  The  skeleton,  the  talking  head,  and  others 
are  thus  exhibited. 

The  extensiveness  of  the,  preparation  for  exhibiting 
the  ghost  will  prevent  most  experimenters  from  at- 
tempting it ;  but  if  the  teacher  would  care  to  show  the 
principle,  he  will  find  the  following  a  cheap  and  effect- 
ive one,  which  he  can  extemporize  with  what  materials 


Fig.  63. 


LIGHT,  8^; 

he  is  likely  to  have  at  hand.  The  beam  of  light  from 
the  porte  lumiere  is  directed  upon  the  object  o^  which 
should  be  a  small  one :  a  doll  dressed  in  white,  or  even 
the  outline  of  one  cut  in  white  paper.  The  light  from 
it  will  of  course  be  scattered  from  it  in  all  directions. 
A  pane  of  white  glass  r  wiil  receive  some  of  these  rays, 
and  reflect  them  toward  j,  where  they  will  appear  to 
come  from  d .  If  the  object  ^  is  a  puppet  or  a  moving 
figure  of  any  sort,  it  can  be  made  quite  a  good  phan- 
tom, though  diminutive.  The  glass  r  can  be  moved  so 
as  to  give  every  one  in  the  room  a  view  of  the  phenom- 
enon, while  the  hand  put  up  to  o'  will  reveal  the  shad- 
owy nature  of  what  is  seen. 

Of  course  all  extraneous  light  should  be  shut  out  by 
having  the  window  curtains  tightly  drawn,  and  also 
with  black  cloth  about  the  apparatus  to  absorb  all  the 
scattered  rays,  especially  about  o  and  o' . 

Obviously,  a  lantern  at  /  could  take  the  place  of  the 
sunbeam,  but  the  light  needs  always  to  be  a  very  strong^ 
one,  for  but  a  fraction  of  the  light  is  reflected  from  the 
object,  and  this  is  again  largely  reduced  by  transmis- 
sion through  the  glass;  nevertheless,  as  the  lig^lt  is 
used  at  the  distance  of  but  a  foot  or  two  from  the  ob- 
ject, it  can  be  lighted  sufficiently  well  for  a  small  room 
in  the  night  with  an  oil  lantern  like  Marcy's  Sciopticon. 


88 


THE  ART  OF  PROJECIING. 


THE    KALEIDOSCOPE. 

The  very  great  beauty  and  variety  of  the  forms  seen 
in  the  kaleidoscope  makes  them  very  desirable  objects 
for  projection.  The  following  method  will  be  found 
efficient : 

I  St.     With  porte  lumiere. 


Fig.  66. 

The  condenser  r  may  have  a  focus  for  parallel  rays 
from  a  foot  to  eighteen  inches  or  more.  Choose  an 
objective  o,  with  focal  length  of  eight  or  ten  inches.  It 
does  not  need  to  be  more  than  an  inch  or  two  in  diam- 
eter. Now  cut  two  strips  of  looking-glass  two  or  three 
inches  broad  and  an  inch  shorter  than  the  focal  length 
of  the  objective.  These  may  have  the  same  breadth 
throughout,  or  they  may  taper  to  an  inch  broad  at  the 
outer  end,  as  shown  in  the  picture.  They  may  now 
have  their  long  edges  brbught  together  on  one  side 
and  inclined  to  each  other  forty-five  or  sixty  degrees, 
and  secured  there  by  enclosure  in  a  tube  ;  or,  if  it  be  for 
temporary  use,  they  may  be  held  in  place  by  a  retort 
clamp  and  work  just  as  well.  The  condenser  c  may 
now  be  placed  close  to  the  orifice  and  its  focus  will 
then  be  at  some  place  o.  Bring  the  fixed  reflectors 
within  the  converging  rays,  so  that  they  will  receive  the 


LIGHT.  89 

focus  just  within  their  outer  and  narrower  endy  and  at 
the  same  time  be  so  inclined  that  the  light  falls  upon 
the  surfaces  of  the  glasses  from  the  broader  end,  as 
shown  above.  Everything  now  depends  upon  the  ad- 
justment of  the  light  to  these  reflectors. 

When  properly  placed,  there  will  appear,  high  upon 
the  screen,  the  sectors  of  the  polygon  equally  illumi- 
nated :  six  of  them,  if  the  reflectors  are  sixty  degrees 
apart,  and  eight,  if  they  are  forty-five  degrees.  No 
direct  light  should  fall  upon  the  screen,  and  will  not,  if 
the  end  of  the  reflectors  be  kept  high  enough  to  cover 
the  focus  of  the  condenser.  A  few  minutes'  work  with 
this  will  enable  one  to  find  the  proper  position  for  the 
best  effect. 

When  the  sectors  appear  equally  illuminated,  the 
objects  to  be  projected  may  be  placed  between  the 
condenser  and  the  reflectors,  —  the  fingers  moved 
about,  a  pencil,  a  key,  a  comb,  or  a  strip  of  paper 
with  pins  in  it,  or  a  leaf  of  a  plant,  perforated  paper, 
or  the  common  glass  trinkets  which  are  usually  put  in 
kaleidoscopes.  If  the  objective  lens  be  put  close  to 
the  outer  end  of  the  reflectors,  the  objects  shown  will 
have  a  much  sharper  outline.  For  the  best  chromatic 
effects  flat  pieces  of  colored  glass  will  be  found  better 
than  round  ones,  as  they  transmit  much  more  light, 
but  an  assortment  of  the  two  will  make  a  fine  ap- 
pearance. 

The  common  kaleidoscopes,  which  are  so  abundant 
in  the  market,  can  be  used  for  this  work  by  removing 
the  ground  glass  in  front  of  them  and  substituting  a 
piece  of  plain  glass.  These  are  generally  provided 
with  a  small  lens,  which  will  answer  for  an  objective, 
but  at  the  end  for  the  eye  there  is  seldom  quite  room 
enough  to  permit  light  to  pass  in  sufficient  quantity  for 


90  THE  ART  aj^'  PROJECTING. 

good  illumination.  By  removing  the  objective  the  dia- 
phragm of  black  paper  can  be  removed.  As  the 
objects  are  all  magnified  so  much,  it  will  be  found  that 
quite  small  bits  of  colored  glass  will  look  better  than 
large  ones. 

2d.  With  a  lantern. 

It  will  be  observed  that  the  essential  condition  for 
showing  the  kaleidoscope  with  the  porte  lumiere  is, 
that  all  the  light  that  reaches  the  screen  must  be  the 
light  that  is  reflected  from  the  inclined  mirrors,  and 
that  the  focus  of  the  converging  beam  must  fall  just  in- 
side the  outer  end  of  them.  Hence  the  focus  needs  to 
be  as  small  as  possible  for  the  best  effect.  With  the 
lime  light  the  focus  is  quite  broad  at  its  narrowest 
part ;  therefore  when  the  kaleidoscope  is  placed  in  the 
beam  it  will  be  necessary  to  adjust  the  light  by  raising 
it,  so  that  the  reflectors  receive  all  of  the  light,  and  it 
also  may  be  necessary  to  draw  it  back  a  little  that  the 
focus  may  come  to  the  proper  place. 

The  ordinary  objective  upon  the  lantern  will  not 
be  needed,  of  course ;  but  an  objective  having  a  focal 
length  equal  to  or  a  little  longer  than  the  length  of  the 
kaleidoscope  may  be  used,  holding  it  in  a  retort 
holder  or  in  any  other  convenient  way.  Let  as  much 
as  possible  of  the  extraneous  light  be  excluded  from 
the  room  by  black  cloth  about  the  front  of  the  lantern. 
With  these  precautions  a  very  good  projection  of  ka- 
leidoscopic forms  can  be  shown.  Even  with  the  better 
forms  of  the  oil  lantern  it  is  possible  to  project  them 
quite  well.  As  the  diameter  of  the  disk  is  doubled 
with  this  fixture,  it  will  be  necessary  to  move  the 
lantern  much  nearer  to  the  screen. 


LIGHT. 


91 


CONCAVE  MIRRORS. 


rig,  67, 


Concave  mirrors,  sufficiently  good  for  demonstration, 
are  fitted  to  wall  lamps,  or  the  reflector  generally  fitted 
to  oil  lanterns  may  be  used.  Such  a  one  held  in  the 
path  of  the  beam  from  the  porte  lumiere  will  reflect 
the  rays  to  a  focus,  where  there  will  be  sufficient  heat 
to  ignite  wood,  paper,  etc.  If  the  mirror  be  tipped, 
the  beam,  after  passing  the  focus,  will  diverge  and 
cover  the  whole  ceiling,  as  the  focus  is  quite  close 
to  the  mirror. 


Fig,  68. 

The  image  formed  in  front  of  the  concave  mirror 
may  be  seen  by  letting  a  strong  light  fall  upon  the 
object  and  having  the  mirror  above  it,  as  indicated. 
If  the  object  0  be  inverted  and  hidden  otherwise  from 
view,  it  will  appear  upright ;  and  at  0,  to  one  standing 
in  front  of  the  mirror,  all  in  a  room  can  be  made  to 
see  it  by  turning  the  mirror  a  little,  so  it  will  face  them. 


92 


THE  ART  OF  PROJECTING, 


A  small  bunch  of  flowers,  a  statuette,  the  hand,  etc., 
are  good  objects  to  exhibit  this  property. 

Images  can  likewise  be  projected  upon  a  screen  by 
means  of  the  concave  mirror. 


Fig,  69. 

At  a  distance  six,  eight,  or  ten  feet  from  a  screen 
hold  a  lighted  candle  close  in  front  of  the  mirror ; 
slowly  separate  them  :  the  image  of  the  light  will 
appear  inverted  upon  the  screen,  and  much  enlarged. 
Advantage  is  taken  of  this  property  of  the  concave 
mirror  to  project  some  phenomena,  such  as  manome- 
iric  flames,  etc.,  which  see. 


CAUSTICS    BY   REFLECTION. 


A  concave  polished  surface,  like  a  strip  of  tin,  two 
or  three  feet  long  and  an  inch  or  two  wide,  bent  into 


Fig.  70, 


LIGHT. 


93 


an  arc  of  a  circle  or  any  other  curve,  held  in  the 
divergent  beam  of  light,  as  shown  in  the  figure,  and 
resting  one  edge  upon  a  white  wall  or  a  piece  of 
white  paper,  will  exhibit  fine  caustic  curves,  which 
will  change  as  the  strip  is  more  or  less  bent.  The 
brighter  the  surface  that  reflects,  the  brighter  will  the 
curves  c  c  appear.  Large  rings,  silvered  and  polished 
on  the  inside,  are  sometimes  used  for  this  ;  but  a  strip 
of  tin  will  answer  well. 

CONVEX    MIRRORS. 

The  back  of  a  concave  mirror,  such  as  already 
mentioned,  forms  a  very  good  convex  mirror.  Hold 
that  in  the  beam  of  light,  in  the  same  way  as  the  con- 
cave mirror  was  held,  and  note  the  result.  Objects  of 
any  size  are  usually  much  distorted  when  seen  by  / 
reflection  in  a  convex  mirror,  as  witness  your  own 
countenance  when  looking  into  one. 

These  distortions  can  be  projected,  though  with 
much  loss  of  light,  by  strongly  illuminating  the  object 
^,  and  with  an  objective  focus  the  reflection  upon  the 


Fig.  71. 


screen.  In  this  way  very  humorous  distortions  of  the 
human  countenance  may  be  photographed  by  using 
the  camera  at  c. 


94 


THE  ART  OF  PROJECTING. 


TOTAL    REFLECTION. 

This  phenomenon  is  generally  shown  by  properly 
directing  a  beam  of  light  into  a  vessel  of  water. 
Perhaps  the  simplest  way  is  to  fill  a  glass  beaker  with 
water,  containing  a  little  milk  or  a  little  magnesia 
stirred  into  it  for  the  purpose  of  enabling  the  eye  to 
trace  the  course  of  the  light  through  it.  Next  placing 
the  beaker  in  a  convenient  place,  with  a  bit  of  looking- 
glass  direct  a  small  beam  of  light  upwards  through 
the  side  of  the  vessel,  so  as  to  strike  the  under  surface 
of  the  water.  By  trial,  the  proper  incident  angle  will 
be  found  at  which  the  light  will  not  emerge  from  the 
upper  surface  of  the  liquid,  but  will  be  totally  reflected  ; 
the  course  of  the  beam  will  be  easily  traced  through 
the  milky  fluid. 

With  suitable  arrangements,  very  striking  and  beau- 
tiful effects  may  be  produced  in   a  stream  of  water. 


Fig.  70. 


The  high  tanks  made  for  showing  the  direction  and 
form  of  water  jets  are  generally  made  with  a  glass 
window  opposite  the  orifice  H,  through  which  a  beam 


LIGHT,  95 

of  light  from  a  lantern  or  from  the  sun  may  be  directed 
while  the  water  flows.  For  the  success  of  this  experi- 
ment it  is  necessary  that  the  orifice  should  be  round, 
smooth,  and  thin,  and  the  body  of  water  in  the  tank 
must  not  be  disturbed  by  currents.  In  the  figure, 
water  is  admitted  at  F,  while  at  G  there  is  a  partition 
with  a  good  many  orifices  in  it  through  which  the  water 
flows,  keeping  it  at  a  constant  height  I.  When,  there- 
fore, the  light  is  concentrated  upon  the  orifice  H,  it  is 
not  scattered,  but  lights  up  the  whole  of  the  curved 
stream,  giving  it  the  appearance  of  molten  silver.  If 
colored  glasses  are  interposed  back  of  E,  D,  the  color 
of  the  stream  will  also  correspondingly  change,  with 
very  pleasing  effects. 

Fig.  71  represents  still  another  form  of  this  experi- 
ment, in  which  the  vertical  attachment  to  the  lantern 
is  used.  A  vertical  fountain  jet  is  opened  in  the 
ascending  beam  from  the  lantern.  The  falling  water 
is  beautifully  illuminated.  Plates  of  colored  glass 
may  be  used,  as  before. 

MIRAGE. 

Direct  the  beam  from  \.)\& porte  lumiere^  so  that  it  is 
horizontal  or  nearly  so.  Put  in  a  diaphragm  with  a 
hole  about  half  an  inch  in  diameter,  or  less,  as  the 
first  condition  is  to  have  a  small  beam  of  parallel  rays. 
No  lens  will  be  needed.  Next  heat  a  brick,  or,  still 
better,  a  poker  or  any  convenient  piece  of  metal  that 
is  a  foot  long  or  more,  until  it  is  nearly  to  a  red  heat ; 
then  place  it  just  in  front  of  the  diaphragm  and  parallel 
with  the  beam  and  about  a  quarter  of  an  inch  below 
it  or  to  one  side  of  it.  The  current  of  heated  air  will 
so  deflect  some  of  the  light  as  to  very  much  elongate 
the  bright  spot  upon  the  screen,  or  even  present 
another  one  some  inches  distant  from  the  first. 


96 


THE  ART  OF  PROJECTING, 


Fig,  71. 


Light. 


97 


REFRACTION. 
I.  —  Of  Glass. 


Fig,  72, 


Project  any  object  that  is  three  or  four  inches  long,  — 
a  lead  pencil  or  an  arrow  cut  out  of  paper.  A  single 
lens  is  all  that  is  needed.  Then  hold  in  front  of  the 
object  a  piece  of  glass  three  or  four  inches  long, 
half  an  inch  broad,  and  the  thicker  the  better.  If  the 
glass  is  held  exactly  perpendicular  to  the  beam  of  light 
no  refraction  will  be  observed  ;  but  turn  one  end  of  it 
towards  the  opening,  and  at  once  the  picture  upon  the 
screen  will  appear  as  if  a  piece  of  the  object  had  been 
cut  out  and  was  held  to  one  side  of  it.  The  thicker 
the  glass  is  the  greater  will  be  the  displacement ;  but  a 
piece  that  is  an  eighth  of  an  inch  will  quite  likely  make 
as  much  difference  as  the  thickness  of  the  object  pro- 
jected. 

Two  pieces  of  glass  may  be  put  together  and  held  as 
before,  or  turned  in  various  directions  with  reference 
to  each  other  and  the  object. 

2  —  Of  Watkr. 

A  hand  mirror  held  at  r  will  reflect  the  light  down- 
ward into  the  chemical  tank  (Fig.  73),  which  should  be 
filled  with  water  in  which  a  little  finely  powdered  resin 

7 


98 


THE   ART  OF  PROJECTING. 


has  been  stirred  to  give  a  turbidity  to  it,  as  the  beam 
will  be  traced  much  better ;  a  few  drops  of  milk  or 
of  chalk-dust  will  answer  the  same  purpose.  A  little 
smoke  in  the  air  will  serve  to  mark  the  course  of  the 


Fig.  73. 

beam  till  it  reaches  the  water,  where  its  direction  will 
be  seen  to  change,  becoming  more  perpendicular.  Re- 
flect the  beam  upon  the  water  at  various  angles  of 
incidence  and  mark  the  course  of  the  refracted  rays. 

In  place  of  the  small  tank  take  a  beaker,  or  any 
vessel  with  square  glass  sides,  and  reflect  the  beam 
upon  the  surface  of  the  water  as  before. 

The  experiment  may  be  varied  by  filling  the  vessel 
half  full  of  water,  then  carefully  pouring  strong  alcohol 
upon  it  to  the  depth  of  an  inch  or  two,  being  careful 
not  to  mix  them  while  pouring,  and  some  ether  upon 
the  alcohol  in  the  same  careful  way.  Sending  now  the 
beam  through  them  as  before,  the  different  refractive 
powers  of  the  various  liquids  will  be  seen. 

3  —  Heathd  Air. 


Fig,  74. 


LIGHT, 


99 


In  the  diverging  beam  from  the  lens  hold  a 
heated  poker  or  any  well  heated  body  at  a.  The 
air  that  is  heated  in  its  neighborhood  will  so  deflect 
the  rays  of  light  as  to  make  quite  a  fine  appearance 
of  heat  upon  the  screen.  An  alcohol  lamp  lighted 
and  held  there  will  have  its  whole  projection  upon 
the  screen :  the  lamp  and  flame  in  outline  shadow, 
while  the  heated  gases  rising  make  an  interesting 
picture. 

To  present  a  still  more  striking  case,  project  a  large 
solar  spectrum,  as  described  on  another  page,  and  about 
half-way  between  the  prism  and  the  screen  hold  the 
lighted  alcohol  lamp,  moving  it  slowly  along  in  the 
different  colors  of  the  spectrum. 

4  —  Lenses. 

The  refractive  power  of  different  forms  of  lenses 
may  be  shown  by  holding  them  in  the  beam  of 
parallel  rays.  The  refractive  power  of  a  lens  of 
water  is  seen  by  taking  two 
wa^ch  glasses  or  one  watch  glass, 
and  a  piece  of  plain  glass  a  little 
larger,  and  bringing  the  two  to- 
gether under  the  surface  of  water. 
The  space  between  these  will  be 
quite  filled  and  they  will  adhere 
tightly  enough  for  the  experi- 
ment. ^^ 

rig,  '75, 


The  same  thing  will  also  be  exhibited   by  placing  a 
watch  glass  upon  the  upper  ring  of  the  vertical  attach- 


loo  THE  ART  OF  PROJECTING. 

ment  (Fig.  27).  When  water  is  poured  in  to  fill  it, 
the  light  will  be  refracted  and  an  object  may  be  pro- 
jected with  it  as  with  any  other  lens. 


Fig.  76, 

The  formation  of  images  by  lenses  is  well  shown  by 
holding  a  lighted  candle  or  lamp  in  front  of  the  screen 
at  any  distance  in  a  darkened  room,  and  bringing  the 
lens  close  to  the  light,  then  moving  it  towards  the 
screen  until  the  inverted  image  appears.  Try  this 
with  double  convex  and  with  plano-convex  lenses  of 
different  focal  lengths,  also  with  a  meniscus  and  with 
a  concave  lens. 

THE    SOLAR    MICROSCOPE. 

This  has  been  described  on  a  former  page,  and  may 
be  turned  to  ;  but,  as  nearly  all  of  the  art  of  projection 
depends  upon  the  use  of  lenses,  it  will  be  well,  in 
giving  instruction  to  dwell  upon  the  conditions  for 
forming  images  with  single  and  with  compound  lenses, 
with  parallel  converging  and  diverging  beams.  The 
porte  lumiere^  the  magic  lantern,  the  solar  microscope, 
the  telescope,  may  be  illustrated  by  methods  that  have 
been  already  explained. 

THE    RAINBOW. 

This  phenomenon  in  nature  is  due  to  refraction  and 
reflection  in  drops  of  water.     It  is  hardly  practicable 


LIGHT.  loi 

to  project  a  rainbow  with  an  artificial  shower,  although 
it  can  be  done  by  having  the  beam  a  widely  diverging 
one,  to  fall  upon  spray  from  a  small  fountain  that 
spreads  a  thin  sheet  at  right  angles  to  it  \  but  a  bow 
that  will  rival  the  natural  one  in  the  sky  may  be  pro- 
jected by  using  two  lenses  with  short  focus,  such  as 
are  put  into  lanterns  for  condensers. 


mg,  77. 


Place  the  first  lens  in  position,  as  if  for  common 
projection.  If  the  second  lens  be  now  brought  near 
the  focus  of  the  former  and  slowly  moved  towards  the 
screen,  a  luminous  disk  will  appear  upon  it,  having  a 
red  border.  Let  this  disk  be  made  as  large  as  is 
desirable,  which  can  be  done  by  moving  the  lens  back- 
ward or  forward.  Now  cut  a  piece  of  paper  r,  with 
a  round  top  a  little  smaller  than  the  diameter  of  the 
lens,  and  place  it  at  r.  All  the  light  from  the  lower 
part  of  the  screen  will  be  cut  off,  and  nothing  will  be 
left  but  a  bow,  with  the  colors  in  the  same  order  as 
those  in  the  primary  bow,  and  very  brilliant.  It  may 
be  enlarged  to  twenty  or  thirty  feet. 

A  second  method  requires  a  conical  prism  ;  and,  if 
this  is  not  already  possessed,  one  may  be  made  by 
taking  a  thin,  clear,  glass  funnel,  with  a  mouth  three 
or  four  inches  in  diameter.     Cut  a  piece  of  plain  white 


I02  THE  ART  OF  PROJECTING, 

glass  a  little  smaller  than  the  mouth  of  the  funnel,  and 
fasten  it  there  with  wax  or  putty  ;  break  off  the  stem ; 
put  the  prism  under  water,  and  when  it  is  filled  stop 
it  with  a  cork  :  it  is  now  ready  for  work. 

Next  cut  out  a  semicircular  slit  from  a  piece  of  paste- 
board. It  need  not  be  more  than  the  one  sixteenth  of 
an  inch  broad^  and  it  may  be  an  inch  across,  as  repre- 
sented at  A,  Fig.  78. 


Vig,  78. 

Place  the  condenser  in  front  of  the  aperture,  and 
hold  the  prism  near  the  focus,  and  then  a  little  distance 
back  of  it  may  be  placed  the  pasteboard  with  the  slit  s. 
The  semicircular  diverging  beam  is  refracted,  and 
suffers  dispersion  ;  which  gives  a  very  good  bow,  but 
with  the  red  innermost,  like  the  secondary  bow. 

Still  another  way  is  possible:  If  a  beam  of  light 
fall  upon  a  cylindrical  reflector,  like  a  glass  rod,  or 
even  a  tin  tube,  like  the  handle  to  a  tin  dipper,  the 
light  is  reflected  from  it  in  a  large  nearly  complete 
circle.  Place  such  a  reflector  at  s  above,  in  place  of 
the  pasteboard  slit,  and  then  with  the  prism  a  bow  will 


LIGHT. 


103 


104  THE  ART  OF  PROJECTING. 

appear,  as  before.  The  prism  may  be  an  ordinary  one, 
but  the  bow  will  not  then  be  perfect  for  a  semicircle. 
The  size  of  this  bow  will  depend  upon  the  size  of  the 
prism,  for  the  ring  of  light  can  be  indefinitely  enlarged 
by  varying  the  angle  of  incidence  of  the  beam. 

With  a  curved  slit  cut  about  the  size  of  a  common 
transparency  and  projected  with  the  lantern,  holding 
a  common  prism  in  front  of  the  objective,  a  very  good 
bow  is  seen.  As  the  refraction  bends  the  rays  down- 
ward, it  will  be  necessary  to  tip  the  front  of  the  lantern 
up  considerable  in  order  to  get  the  bow  upon  the 
screen.     (Fig.  79.) 

Lastly,  let  a  beam  of  parallel  rays,  about  an  inch  in 
diameter,  fall  upon  a  glass  sphere  filled  with  water,  — 
an  ordinary  small  glass  flask  answers  well,  but  the 
larger  the  flask  the  greater  should  be  the  size  of  the 
beam.  If,  now,  a  small  white  screen  be  placed  between 
the  sphere  and  the  aperture  in  the  window,  a  bow  will 
be  seen  concentric  with  the  aperture  and  arranged  so 
that  the  red  is  outside  and  the  violet  inside.  At  a 
greater  distance  from  the  aperture  another  bow  will  be 
formed,  much  fainter  than  the  first,  and  with  the  colors 
in  the  inverse  order.  If  diflerent  colored  glasses  are 
interposed  in  the  path  of  the  beam  of  white  light,  the 
bow  will  be  seen  to  consist  mainly  of  the  tint  of  the 
glass. 

CHROMATIC   ABERRATION. 

Caustic  curves,  due  to  chromatic  aberration  in  the 
lenses,  may  be  projected  by  taking  two  rather  large 
lenses  of  short  focus,  such,  for  instance,  as  those  made 
for  lantern  condensers. 

Place  the  first  one  as  if  for  common  projections. 
The  second  may  be  held  in  the  hand  and  brought  near 
to  the  focus  of  the  first  and  then  inclined,  as  shown  in 


LIGHT.  105 

the  figure.  Moving  it  towards  the  screen,  beautiful 
colored  figures  will  appear,  which  will  change  with  the 
angle  of  the  second  lens  to  the  light  that  is  incident 
upon  it.     A  comet,  a  hollow  funnel,  a  mock  sun,  and 


Fig.  80. 

Other  curious  forms  may  be  projected,  all  of  them 
brilliantly  colored. 

With  a  lantern,  it  will  be  sufficient  to  remove  the 
objective  and  place  a  large  lens,  like  one  of  the  above, 
near  the  focus  of  the  condenser,  when  the  same  figures 
will  appear. 

DISPERSION 

Is  usually  shown  by  decomposing  a  beam  of  solar  light 
with  a  triangular  prism.  The  beam  should  be  a  rather 
small  one,  not  more  than  one  fourth  of  an  inch  in 
diameter,  if  a  pure  spectrum  be  wanted.  If  it  be  more 
than  this,  there  will  be  more  or  less  white  light  in  the 
middle  of  the  band. 


Fiff.  SI. 

The  smaller  the  aperture,  the  purer  will  be  the  colors 
into  which  the  light  has  been  decomposed ;  but  if  a 


io6 


THE  ART  OF  PROJECTING, 


very  small  beam  be  used,  the  room  will  need  to  be 
quite  dark.  When  it  is  desirable  to  have  a  large  and 
brilliant  spectrum,  the  light  may  be  sent  through  a 
condenser,  with  a  focus  one  or  two  feet  long,  and  using 


Fig,  82, 

a  diaphragm  near  the  focus  to  cut  off  the  marginal 
rays.     This  will  permit  much  more  light  to  be  properly 

refracted  for  a  good 
spectrum.  At  a 
distance  of  twenty- 
five  feet,  a  common 
triangular  prism 
will  give  a  spec- 
Fig,  S3,  trum      about     five 

feet  long;  but  it  may  be  indefinitely  lengthened  by 
inclining  the  screen  to  it,  as  shown  above,  and  it  will 
usually  be  quite  bright  when  made  ten  or  fifteen  feet 
long ;  if  the  room  be  otherwise,  quite  dark. 

The  dispersive  power  of  different  substances  may  be 
shown  by  making  a  V  trough  of  glass,  with  an  included 
angle  of  sixty  degrees.  Water,  alcohol,  ether,  spirits 
of  turpentine,  etc.,  may  be  put  in  it,  and  the  beam  sent 
through  them.  In  this  case  the  spectrum  will  appear 
overhead  upon  the  ceiling.  If  the  glass  trough  have 
three  or  four  tight  partitions  in  it,  all  the  substances 
may  be  used  .at  once,  and  thus  their  refractive  powers 
compared. 


LIGHT.  107 

COLORS    OF   THIN    FILMS. 

Let  a  soap  bubble  be  held  in  the  beam  of  diverging 
rays,  near  the  focus  of  the  lens  (Fig.  74),  and  in  such 
a. position  that  some  of  the  light  will  be  reflected  from 
its  upper  surface. 

As  soon  as  the  bubble  becomes  thin  enough,  brilliant 
colors  will  appear  upon  it,  which  will  be  reflected  to 
the  walls  and  ceiling,  as  they  will  spread  over  a 
large  surface.  If  the  bubble  is  held  quiet  long  enough, 
each  cf  the  prismatic  tints  will  appear  in  turn  upon  the 
walls,  and  sometimes  the  series  will  be  repeated. 

If  the  bubble  is  projected  in  the  way  mentioned 
upon  page  44,  three  or  four  of  these  series  may  be  seen 
at  the  same  time. 

Instead  of  blowing  a  bubble  with  a  pipe,  as  shown 
in  that  figure,  blow  a  mass  of  them  in  the  dish  con- 
taining the  solution.  Very  large  masses  may  be  made 
and  the  colors  reflected  from  them  in  the  same  way  as 
above,  or  with  the  lantern. 

The  tension  of  the  bubble  film  may  be  shown  by 
leaving  the  tube  open  after  the  bubble  is  blown,  when 
the  latter  will  contract  as  if  it  were  being  drawn  into 
the  bowl  of  the  pipe  ;  or  the  bubble  may  be  blown  upon 
the  end  of  a  glass  tube 
bent  twice  at  right  an- 
gles, after  which  the 
open  end  may  be  put 
an  inch  or  two  under 
the  surface  of  water  in 
the  chemical  tank  and  jrig,  85. 

projected.     The  water  in  the  tube  will  stand  below  the 
level  of  the  water  in  the  tank  indicating  pressure. 

When  these  colors  from  thin  films  appear  upon  the 


lo8  THE  ART  OF  PROJECTING. 

screen,  pieces  of  glass  of  various  colors  may  be  inter 
posed  between  the  lens  and  the  bubbles,  when  dark  or 
black  bands  will  be  seen  to  take  the  place  of  those 
colors  that  have  been  stopped  by  the  tinted  glass. 

Yellow  light  that  is  nearly  monochromatic  can  be 
obtained  by  interposing  a  crystal  of  bichromate  of 
potash.     Let  the  crystal  be  a  thin  and  quite  clear  one. 

Colored  solutions  may  be  used  for  the  same  purpose. 

Under  the  head  of  Spectrum  Analysis  other  means 
for  producing  monochromatic  light  will  be  found,  with 
colored  lights  which  are  appropriate  for  examining 
bubbles. 

Bubbles  made  of  common  soap-suds  will  not  last 
long,  and  various  preparations  have  been  described 
for  making  persistent  bubbles,  some  of  which  would 
last  three  days. 

A  piece  of  glycerine  soap  about  the  size  of  a  marble, 
sliced  and  dissolved  in  water  at  a  iio°  Fah.,  will  make 
a  bubble  that  will  last  half  an  hour.  Prof.  Cooke  gives 
the  following  method  for  making  a  still  more  persistent 
bubble :  — 

"  Procure  a  quart  bottle  of  clear  glass,  and  some  of 
the  best  white  castile  soap  (or,  still  better,  pure  palm- 
oil  soap).  Cut  the  soap  (about  four  ounces)  into  thin 
shavings,  and  having  put  them  into  the  bottle  fill 
this  up  with  distilled  or  rain  water,  and  shake  it 
well  together.  Repeat  the  shaking  until  you  get  a 
saturated  solution  of  soap.  If,  on  standing,  the  solu- 
tion settles  perfectly  clear,  you  are  prepared  for  the 
next  step  ;  if  not,  pour  off  the  liquid  and  add  more 
water  to  the  same  shavings,  shaking,  as  before.  The . 
second  trial  will  hardly  fail  to  give  you  a  clear  solution. 
Then  add  to  two  volumes  of  soap  solution  one  volume 
of  pure    concentrated  glycerine. 


LIGHT.  109 


NEWTON'S    RINGS. 


Choose  a  piece  of  white  window-glass  three  or  four 
inches  square,  and  witji  clothes-pins  or  other  means 
clamp  it  to  the  lens  with  longest  focus  you  have  ;  a 
lens  with  focal  length  of 'two  or  three  feet  will  answer, 
though  less  curvature  is  better.  Find  by  rocking  the 
lens  upon  the  plate  with  the  thumbs -where  the  point  of 
contact  is.  This  may  be  seen  by  a  set  of  rings  which 
surround  it,  and  which  move  from  place  to  place  when 
the  lens  is  rocked.  Having  found  this  place  where 
the  rings  appear,  place  it  near  the  focus  of  the  con- 
denser having  a  diaphragm  of  pasteboard  with  a  hole 
in  it  not  more  than  a  quarter  of  an  inch  in  diameter 
just  back  of  the  plate.  This  cuts  off  most  of  the 
light  that  would  otherwise  be  scattered  in  the  room,  and 
prevents  the  rings  from  appearing  plain.  The  objective 
used  may  have  an  inch  focus.  There  will  usually 
be  seen  as  many  as  six  rings,  and  the  outer  ones  at 
the  distance  of  twenty  feet  or  more  may  be  two  or 
three  feet  in  diameter.  By  interposing  colored  glasses 
or  colored  solutions,  as  with  the  bubbles,  these  colored 
rings  will  appear  alternately  with  black  rings. 


RECOMPOSITION   OF   WHITE   LIGHT. 

This  may  be  effected  in  several  ways. 

ist.  By  receiving  the  decomposed  light  from  one 
prism  upon  the  face  of  another  prism  like  it,  but  turned 
so  that  the  ray  will  have  its  original  direction. 

2d.  By  a  lens.  Let  the  decomposed  rays  from  the 
prism  fall  upon  a  double  convex  lens  placed  so  near  to 
the  prism  that  all  of  the  colors  of  the  spectrum  may 
pass  through  it.  .  Bring  the  screen   to  the  conjugate 


no 


THE  ART  OF  PROJECTING. 


focus  of  the  lens,  and  then  the  light  will  appear  as  a 
brilliant   white   spot.      Interpose    a  piece    of  colored 

j  glass,  and  the  spot  will 
l-at  once  change  its  color. 
3d.  By  reflection  from 
a  concave  mirror.     The 
colored  rays  will  be  con- 
Fig,  86,  verged    as   white    light 

would  be,  and  appear  upon  a  small  screen  placed  at  the 
focus  as  a  spot  of  white  light 
4th.  By  reflection  from  a 
series  of  small  mirrors.  Let 
the  spectrum  fall  upon  the 
small  mirrors,  and  so  incline 
them    that  they  will  reflect  Pig^87. 

the  light  to  the  same  place  upon  the  screen  or  the  wall. 
5th.  By  rotating  colored  disks. 
Disks  painted  with  the  colors  of  the  spectrum  are 
sold  in  the  market  under  the  name  of  Newton's  disks. 
They  are  made  by  pasting  sections  of  colored  tissue 
paper  upon  a  large,  stiff  pasteboard  disk. 

These   colors   should   have    the    following   angular 
value  :  — 


Red,  60°, 
Orange,  35% 
Yellow,  55°, 
Green,  60°, 


Blue,  55°, 
Indigo,  35°, 
Violet,  60°. 


This  disk  may  be  rotated  upon  the  whirling  table, 
or,  what  is  much  better,  a  zoetrope  rotator,  and  it  will 
appear  a  dusky  white.  It  will  be  better  to  have  a  strong 
light  thrown  upon  it  while  it  is  turning. 

Another  good  way  is  to  cut  disks  of  properly-colored 
papers  and  make   a  radial   slit  in  them.     When  put 


LIGHT.  1 1 1 

upon  the  rotator,  they  can  be  made  to  slide  by  each 
other  so  as  to  expose  a  greater  or  less  angle  of  any 
color.  By  using  any  two  or  more  of  these  at  a  time, 
many  interestino^effects  from  combined  colors  can  be 
exhibited. 

One  may  often  find  colored  stars  or  rings  or  other 
fanciful  designs  on  posters  for  advertisements  or  wrap- 
pings on  goods  of  various  sorts,  which  may  be  utilized 
with  the  rotator  in  studying  color, 

fraunhofer's  lines. 

The  solar  spectrum  as  usually  projected  with  around 
orifice  and  common  prism,  with  an  included  angle  of 
60°,  appears  complete,  and  is  often  called  a  pure  spec- 
trum. If,  however,  the  prism  be  of  flint-glass  or,  better 
still,  a  bottle  prism  filled  with  bisulphide  of  carbon,  it 
may  be  placed  in  such  a  position  as  to  present  the 
absorption  lines  known  as  Fraunhofer's. 


Fif7. 

To  do  this  it  will  only  be  necessary  to  place  the 
prism  in  the  full  beam- from  the  porte  lumiire  znd  turn 
it  so  that  one  side  is  very  nearly  parallel  with  the  beam. 
A  spectrum  will  be  formed  containing  a  number  of 
dark  perpendicular  lines  known  as  the  C  D  E  F  and 
G  lines.     These  may  be  still  more  marked  by  placing 


112  THE  ART  OF  PROJECTING. 

a  lens  in  front  of  the  orifice  at  about  its  focal  length 
distant  from  it,  and  placing  the  prism  at  its  focus,  and 
inclined  to  the  concentrated  beam  in  the  same  way  as 
above.  The  spectrum  will  then  be  very  bright  and 
some  lines  well  marked. 

In  order  to  show  the  Fraunhofer  lines  to  advantage 
it  is  necessary  to  have  the  room  quite  dark  ;  to  use  a 
very  narrow  slit  and  a  lens  in  conjunction  with  a  good 
triangular  prism  of  flint  glass  or  of  bisulphide  of 
carbon.  The  diaphragm  containing  the  slit  through 
which  the  light  must  pass-should  be  placed  close  to  the 
opening  in  order  to  exclude  all  the  light  that  is  not 
directly  used. 


Fig^  89. 

This  diaphragm  may  be  made  of  pasteboard  with 
a  slit  cut  in  it  three  quarters  of  an  inch  long  and 
the  fiftieth  of  an  inch  in  width  ;  the  edges  should  be 
smooth  and  parallel.  A  lens  with  a  focus  of  five  or 
six  feet  is  best  for  sharp  definition  of  the  lines,  but 
one  with  a  focus  of  only  a  foot  or  two  may  be  used  to 
exhibit  the  large  and  more  prominent  of  them.  Place 
the  lens  at  such  a  distance  from  the  slit  as  to  project 
it  sharply  upon  the  screen,  at  a  distance  from  the  lens, 
say  twenty  feet.    Then  bring  the  triangular  prism  c/os^ 


LIGHT.  113 

to  the  lens  as  shown  :  The  light  will  be  deflected  and 
dispersed,  and  the  screen  should  now  be  brought 
where  the  spectrum  will  fall  perpendicularly  upon  it, 
and  at  the  same  distance  from  the  lens  that  it  was 
before,  namely,  twenty  feet.  Turn  the  prism  until  the 
spectrum  has  its  least  deviation,  which  will  be  found  by 
a  little  trial.  The  Fraunhofer  lines  should  appear.  If 
they  are  indistinct,  move  both  the  lens  and  prism  back 
or  forward  in  the  beam  until  they  are  distinct,  for  it  is 
now  only  a  matter  of  focussing. 

If  the  lens  has  a  focus  five  or  six  feet  distant  it  will 
need  to  be  quite  as  far  from  the  slit  as  the  length  of 
its  focus,  and  the  screen  adjusted  as  before,  but  the 
lines  should  appear  plainer  and  in  greater  number. 
With  such  a  lens  and  a  good  glass  prism  the  spectrum 
should  be  about  five  feet  long,  and  with  good  focussing 
the  D  line  should  be  seen  double.  These  lines  may 
be  seen  by  a  large  number  by  moving  the  screen  edge- 
wise an  inch  or  two. 

One  may  use  a  condenser  and  converge  a  large  beam 
upon  the  slit.  This  will  make  the  spectrum  brighter 
and  permit  a  narrower  slit  to  be  used,  but  the  definition 
of  the  lines  is  not  so  good  as  when  parallel  rays  fall 
upon  the  lens.  If  the  object  be  to  project  a  spectrufti 
that  shall  be  well  defined  upon  its  sides  and  to  show 
only  the  more  prominent  lines,  let  the  slit  be  made  as 
broad  as  the  twentieth  of  an  inch  ;  a  lens  with  about  a 
foot  focus  may  be  used  to  project  the  slit  in  the  ordi- 
nary way,  and  the  prism  placed  at  the  focus  and  turned 
to  its  angle  of  least  deviation,  which,  as  before,  must  be 
found  by  trial.  In  this  way  a  beautiful  and  well-defined 
spectrum  will  be  produced,  which  at  the -distance  of 
twenty  feet  would  be  about  five  feet  long  and  two  feet 
broad. 

8 


114  THE  ART  OF  PROJECTING. 

ABSORPTION    BANDS. 

If  a  piece  of  colored  glass  be  held  in  the  path  of 
the  beam  of  white  light  before  it  enters  the  lens,  Fig. 
89,  a  part  of  the  light  will  be  absorbed  and  black 
bands  of  greater  or  less  breadth  will  appear  upon  the 
screen.  The  glass  may  be  held  between  the  prism 
and  the  screen  with  about  the  same  result.  Some 
of  the  pieces  of  colored  glass,  which  are  quite  com- 
mon, will  give  very  distinct  absorption  bands.  It  will 
be  well  to  try  red,  yellow,  green,  blue,  and  violet 
glasses.  If  the  color  is  very  deep  a  greater  width 
must  be  given  to  the  slit  else  the  spectrum  will  be  seen 
with  difficulty. 

The  chemical  tank  (see  page  34)  may  be  used  to 
hold  solutions  of  various  kinds  in  this  place.  A  wedge- 
shaped  tank  is  also  very  convenient,  as  it  enables  one 


Fig,  90. 

to  pass  the  light  through  any  i?equired  thickness  of  a 
solution,  and  thus  to  note  the  effects  of  thickness  upon 
absorptive  action.  This  tank  may  be  made  five  inches 
long,  four  inches  broad,  and  an  inch  thick  at  its  broad 
end.  A  piece  of  thick  rubber  cut  as  in  the  figure  will 
answer  for  bottom  and  edges  of  this  tank. 


LIGHT.  115 

Each  end  being  bent  up  at  right  angles,  the  glass 
may  be  bound  to  it  by  clamps,  as  in  the  other  tank. 

"  A  solution  of  alizarin  in  carbonate  of  potassium 
or  sodium,  or  in  ammonia,  exhibits  a  spectrum  having 
a  band  of  absorption  in  the  yellow,  another  narrower 
one  between  the  red  and  the  orange,  and  a  third  very 
inconspicuous  band  coinciding  with  the  line  E.  Pur- 
purine  dissolved  in  carbonate  of  potassium  or  sodium 
exhibits  two  dark  bands  of  absorption  about  the  green 
part  of  the  spectrum.  A  solution  of  the  same  sub- 
stance in  aqueous  alum  exhibits  the  same  peculiar  mode 
of  absorption,  but  likewise  a  yellow  fluorescence.  A 
solution  of  purpurine  in  sulphide  of  carbon  exhibits 
four  bands  of  absorption,  of  which  the  first,  situated 
in  the  yellow  just  beyond  D,  reckoning  from  the  red 
extremity,  is  narrower  than  the  rest.  The  second  is 
situated  in  the  green,  nearly  coinciding  with  the  line 
E,  The  third  in  the  blue,  near  F,  and  the  fourth, 
which  is  very  inconspicuous,  in  the  indigo.  Lastly,  the 
solution  of  purpurine  in  ether  gives  a  spectrum  giving 
two  bands  of  absorption,  one  narrow  and  very  dark  in 
the  green,  nearly  coinciding  with  E.  The  second  in 
the  blue,  broader  and  less  strongly  marked,  and  having 
its  centre  at  the  line  F ;  the  solution  is  also  slightly 
fluorescent."     (Stokes.) 

The  following  series  of  experiments  upon  Absorption 
is  taken  from  an  article  by  A.  H.  Allen  in  Nature,  vol. 
4,  p  346.  A  lime  light  may  be  used  if  it  is  desirable 
to  project  these  when  sunlight  is  not  available  :  — 

"  A  beam  of  light  from  the  lantern  is  passed  through 
a  slit,  focussed  by  a  lens,  refracted  by  a  bisulphide  of 
carbon  prism,  and  the  spectrum  exhibited  in  the  usual 
way.  A  flat  cell  containing  a  solution  of  permanga- 
nate of  potash  is  next  placed  in  front  of  the  slit.    With 


Il6  THE  ART  OF  PROJECTING. 

a  weak  solution  and  narrow  slit  a  series  of  black  bands 
are  produced  in  the  green  part  of  the  spectrum ;  but 
with  a  stronger  solution  the  green  and  yellow  are  com- 
pletely cut  out,  allowing  only  the  red  and  deep  blue 
lights  to  pass.  On  widening  the  slit  these  bands  of 
colored  light  of  course  increase  in  width  also,  gradually 
approaching  each  other  until  they  overlap,  producing  a 
fine  purple  by  their  adniixture. 

If  the  experiment  be  repeated,  substituting  for  the 
permanganate  an  alkaline  mixture  of  litmus  and  potas- 
sium chromate  in  certain  proportions,  only  the  red  and 
green  light  are  transmitted,  the  blue,  and  especially 
the  yellow^  being  completely  absorbed. 

On  widening  the  slit  as  before,  the  red  and  green 
bands  overlap  and  produce  by  their  union  a  very  fine 
compound  yellow,  while  the  constituent  red  and  green 
are  still  visible  on  each  side.  The  effect  is  most  strik- 
ing when  by  the  widening  of  the  slit  a  round  hole  is 
exposed  in  its  place,  when  then  appear  on  the  screen 
two  circles,  respectively  green  and  red,  producing 
bright  yellow  by  their  mixture.  This  experiment  is  the 
more  striking  as.  it  immediately  follows  the  process 
of  absorbing  the  simple  yellow.  The  mixture  above 
described  (suggested  by  Mr.  Strull)  answers  better  than 
a  solution  of  chromic  chloride. 

Of  course,  it  is  a  well-known  fact  that  all  natural 
yellows  give  a  spectrum  of  red,  yellow,  and  ^een,  and 
a  common  effect  illustrating  the  compound  nature  of 
yellow  is  noticed  when  exhibiting  a  continuous  spec- 
trum on  a  screen.  When  the  slit  is  narrow  the  green 
is  very  fully  developed  and  only  separated  from  the 
red  by  a  very  narrow  strip  of  yellow,  while  on  gradually 
increasing  the  width  of  the  slit  the  red  and  green  are 
9Ure   to  overlap,   producing    the    brilliant  yellow   we 


LIGHT.  117 

generally  notice.  Thus  the  purer  the  spectrum  the 
less  yellow  is  observed. 

If  the  continuous  spectrum  be  produced  with  a  quartz 
prism,  a  little  management  and  adjustment  of  distance 
of  the  screen  will  cause  the  two  spectra  to  overlap  so 
that  the  red  of  one  may  Se  made  to  coincide  with  the 
green,  blue,  or  any  desired  tint  of  the  other.  The 
same  result  is  obtained  by  employing  two  slits  at  the 
same  time,  the  distance  between  which  can  be  adjusted. 
By  this  means  two  spectra  are  obtained  simultaneously, 
any  portions  of  which  may  be  made  to  coincide. 

A  saturated  solution  of  potassium  chromate  absorbs 
all  rays  more  refrangible  than  the  green,  while  a  solu- 
tion of  ammonio,  sulphate  of  copper  stops  all  but  the 
blue  and  green.  These  statements  may  be  proved  by 
placing  flat  cells  containing  the  liquids  in  front  of  the 
slit  of  the  lantern,  and  on  placing  one  cell  in  front  of  the 
other  in  the  same  position,  the  green  light  only  is  trans- 
mitted. This  experiment  serves  to  explain  the  reason 
that  the  mixture  of  yellow  and  blue  generally  results 
in  green,  all  other  rays  being  absorbed  by  one  or  other 
of  the  constituents. 

By  placing  the  two  cells  in  front  of  separate  lanterns 
and  throwing  disks  of  light  upon  the  screen,  a  beauti- 
fully pure  white  is  produced  when  the  blue  and  yellow 
overlap.  I  employ  one  lantern  only  for  this  exper- 
iment, using  two  focussing  lenses  side  by  side  to  pro- 
duce the  overlapping  circles  of  light.  I  also  employ 
a  cell  with  three  compartments,  containing  solutions  of 
analine,  ammonio,  sulphate  of  copper,  and  a  mixture 
of  potassium  chromate  with  the  last  solution,  and  pro- 
jecting images  on  the  screen  by  means  of  three  lenses 
fitted  on  the  same  stand  but  capable  of  separate  ad- 
justment. 


Ii8  THE  ART  OF  PROJECTING. 

I  can  thus  exhibit  overlapping  circles  of  brilliant 
red,  blue,  and  green  light,  which  produce  a  perfect 
white  by  their  admixture  ;  while  at  the  same  time  there 
is  seen  the  compound  yellow  produced  by  the  union  of 
red  and  green,  the  purple  arising  from  the  red  and 
blue,  and  a  color  varying  from  grass  green  to  sky  blue 
produced  by  the  combination  of  the  green  and  blue 
light.  This  experiment  has  the  advantage  of  exhibit- 
ing at  the  same  time  the  three  primary  colors,  —  red, 
gree?i,  and  blue,  —  the  compound  colors  produced  by 
their  mixture,  their  complimentary  lints,  and  the  syn- 
thesis of  white  light." 

The  flat  cells  mentioned  are  made  by  cutting  thin 
pieces  of  board  to  the  desired  shape,  and  cementing 
pieces  of  window-glass  on  each  side  by  means  of  pitch. 

INTERFERENCE   SPECTRA   IN    REFLECTED    LIGHT. 


Fig,  91. 

Let  a  beam  of  light  about  an  inch  in  diameter  fall 
upon  a  thin  piece  of  mica,  M,  distant  eight  or  ten  feet 
from  the  porfe  lumiere.  A  part  of  the  light  will  be 
reflected,  and  in  that  may  be  placed  a  slit  at  Z,  and  a 
lens  O  may  project  the  slit  in  the  ordinary  way.  At 
the  focus  of  the  lens  place  a  good  prism  so  as  to  have 


LIGHT,  119 

a  spectrum  fall  upon  the  screen  at  S.  This  spectrum 
will  be  seen  to  be  traversed  by  a  large  number  of  black 
bands  distributed  throughout  the  whole  length  of  it. 
If  the  plate  of  mica  be  very  thin  and  white  there  may 
be  as  few  as  eight  of  these  striae,  but  if  it  be  thicker 
their  number  will  be  largely  increased. 

The  room  will  need  to  be  made  as  dark  as  possible 
for  this  experiment,  as  the  spectrum  will  not  be  very 
bright  at  best,  and  it  therefore  cannot  be  enlarged. 
If  the  length  of  the  spectrum  exceeds  a  foot  it  will  be 
quite  dim.  These  lines,  however,  can  be  seen  to  great 
advantage  by  placing  the  eye  close  to  the  prism  when 
in  its  place  as  shown  above. 

If  the  spectrum  of  the  light  reflected  from  mica  be 
received  upon  a  paper  screen  painted  over  with  a  solu- 
tion of  quinine  and  thus  rendered  fluorescent,  such 
interference  striae  will  make  their  appearance  in  the 
ultra  violet  part  of  the  spectrum. 

SPECTRUM   ANALYSIS. 

Ta  project  the  spectrum  of  any  substance  what- 
ever it  must  be  heated  until  its  vapor  is  brilliantly 
incandescent.  The  heat  of  the  electric  arc  is  best 
for  this  work  as  every  substance  is  vaporized  there. 
The  lime  light  may  be  used  to  exhibit  the  prin- 
ciples of  spectrum  analysis,  but  its  heat  is  insuffi- 
cient for  most  of  the  metals.  The  characteristic 
lines  of  Sodium,  Calcium,  Lithium,  Barium,  Stron- 
tium, Potassium,  and  Copper  may  be  tolerably  well 
exhibited  with  a  lantern  furnished  with  oxyhydrogen 
jet  and  gases. 

I  St.     To  exhibit  the  spectrum  :  — 

Produce  the  lime  light  as  you  would  for  common 
projection.     Remove  the  objective  and  place  at  the 


I20  THE  ART  OF  PROJECTING. 

focus  in  front  of  the  lantern  the  slit  d.  The  objective 
0  may  then  be  so  placed  as  to  project  a  sharp  image  of 
the  slit  upon  a  screen  in  front  of  it  at  a  distance  of 
fifteen  or  twenty  feet ;  then   place  the  triangular  prism 


Fig.  92, 

close  to  the  objective.  The  screen  v^^ill  now  need  to 
be  moved,  that  the  refracted  rays  may  fall  upon  it,  and 
at  the  same  distance  from  the  objective  that  it  stood 
in  front,  otherwise  the  edges  of  the  spectrum  will 
appear  blurred.  This  should  give  a  spectrum  about 
five  feet  long  at  the  distance  of  twenty  feet,  but  the 
length  will  depend  upon  the  dispersive  power  of  the 
prism.  It  will  be  longer  with  a  bisulphide  of  carbon 
prism  than  with  one  made  of  glass.  If  a  still  longer 
one  is  needed  use  two  similar  prisms  close  together 
and  each  one  turned  to  the  point  of  minimum  devia- 
tion. 

If  a  very  pure  spectrum  is  needed,  all  of  the  con- 
densers may  be  removed  and  the  slit  put  in  their  place. 
A  parallel  beam  will  then  fall  upon  it,  and  the  projec- 
tion may  then  be  made  in  precisely  the  same  way  as 
for  the  solar  spectrum.  In  this  case  the  light  will  be 
much  less  intense. 

2d.     To  project  the  spectrum  of  the  elements :  — 
Remove  the  lime  cylinder  and  its  holder,  and  light 
the  gases :  the  tongue  of  flame  will   be  six  or  eight 


LIGHT,  121 

inches  long.  Now  hold  a  stick  of  glass  like  a  large  glass 
stirring  rod  in  the  flame  at  the  same  place  when  the 
lime  cylinder  is  fixed:  It  will  glow  brilliantly  with 
nearly  the  monochromatic  light  of  sodium,  and  if  the 
prism  is  in  its  place  the  bright  yellow  line  indicative  of 
that  element  will  appear  upon  the  screen.  The  glass 
will  need  to  be  turned  slowly,  and  the  attention  of  one 
person  will  be  needed  constantly  to  keep  it  in  place. 
Sticks  of  soda  glass  may  be  had  in  the  market,  made 
especially  for  projecting  the  sodium  line  in  this  way, 
but  the  spectrum  can  be  obtained  from  almost  any 
piece  of  glass. 

Another  good  method  is  to  soak  soft-pine  sticks  six 
or  eight  inches  long  and  half  an  inch  thick  in  saturated 
solutions  of  the  chlorides  of  the  various  elements  to  be 
projected,  as  the  chlorides  are  more  volatile  than  other 
salts.  Let  the  sticks  remain  in  these  solutions  several 
days  before  they  are  to  be  used,  as  a  much  larger 
quantity  of  the  material  will  be  absorbed.  These 
solutions  may  conveniently  be  made  in  test  tubes  six 
or  eight  inches  long,  remembering  to  label  each  tube 
by  pasting  a  bit  of  paper  upon  it  and  writing  the  symbol 
of  the  substance  contained  in  it.  The  chlorides  of  all 
the  substances  named  above  may  be  prepared  in  this 
way  and  a  stick  provided  for  each  one. 

The  saturated  and  still  wet  stick  must  be  put  imme- 
diately into  the  flame  where  the  glass  and  the  lime 
cylinder  are  otherwise  placed,  and,  holding  one  end  in 
the  hand,  keep  turning  it  slowly.  The  stick  will  glow 
and  give  out  the  kind  of  light  that  is  peculiar  to  the 
included  element. 

The  spectrum  consisting  of  bright  lines  will  be 
quite  bright  and  sufficiently  large  to  be  plainly  seen  by 
an  audience  of  a  thousand  persons.    Sodium,  Calcium, 


122  THE  ART  OF  PROJECTING. 

Lithium,  and  Copper  are  especially  good  for  this  work 
and  give  satisfactory  spectra. 

When  this  monochromatic  light  from  the  stick  of 
glass  or  the  saturated  solution  of  sodium  chloride  is 
made  to  appear,  it  will  be  a  good  time  to  give  atten- 
tion to  its  effects  upon  other  colors.  Observe  the 
faces  of  individuals,  the  colors  of  flowers,  of  ribbons, 
of  pictures.  It  is  a  good  plan  to  have  prepared  a  set 
of  strips  of  bright-colored  papers,  or  ribbons,  or  the 
Newton's  disk,  for  exhibition  in  monochromatic  light. 

REVERSED   LINE. 

The  dark  sodium  line  is  the  only  one  that  is  ever 
projected,  owing  to  the  great  difficulty  there  is  in 
making  the  vapors  of  other  substances  sufficiently 
dense  to  absorb  the  powerful  rays  from  the  electric  arc 
or  of  the  lime  light.  With  either,  a  pure  spectrum 
must  first  be  projected,  and  the  slit  should  be  nicely 
focussed,  as  described.  —  Then  having  provided  a 
gas  jet  with  Bunsen  burner,  or  an  alcohol  lamp  in 
front  of  the  slit,  hold  in  it  a  small  iron  spoon  con- 
taining a  lump  of  metallic  sodium  as  large  as  a  pea. 
It  will  take  fire  and  burn  with  a  yellow  blaze  and  a 
white  vapor,  through  which  the  light  from  the  lantern 
must  pass.  If  this  vapor  is  dense  enough  it  will  stop 
rays  from  the  other  light  that  have  the  same  refrangi- 
bility ;  and  as  its  own  luminousness  is  not  very  great, 
it  will  leave  a  black  line  upon  the  screen  in  the  place 
where  the  sodium  line  would  appear  if  the  light  came 
from  it. 

It  will  be  best  to  have  a  screen  a  foot  square  with  a 
hole  through  it,  to  set  in  front  of  the  sodium  flame  to 
prevent  its  light  from  falling  upon  the  large  screen  and 
injuring  the  effect. 


LIGHT. 


123 


FLUORESCENCE. 

Only  blue  or  violet  or  ultra-violet  rays  are  capable  of 
producing  this  phenomenon,  and  these  may  be  obtained 
either  by  passing  common  white  light  from  the  sun,  or 
the  electric  light,  or  the  lime  light,  through  a  piece  of 
blue  or  violet  glass  or  through  a  solution  of  ammonia, 
sulphate  of  copper;  or,  better  still,  by  producing  a  pure 
spectrum.  The  best  effects  are  to  be  observed  by  using 
a  prism  of  great  dispersive  power,  like  quartz. 


Fig,  93, 


When  colored  glass  is  to  be  used  to  obtain  the 
violet  light,  it  suffices  to  place  a  lens  of  a  foot  focus 
near  the  orifice  and  the  glass  just  in  front  of  it. 
Fluorescent  solids  and  solutions  may  then  be  examined 
at  S.  A  piece  of  uranium  glass  or  a  solution  of  quin- 
ine in  a  test  tube  or  bottle  will  exhibit  this  property 
so  that  many  can  see  it  at  the  same  time.  It  will  be 
well  to  use  two  bottles  or  beakers  of  clear  glass,  —  one 
to  contain  pure  water  and  the  other  the  solution  of 
quinine  —  and  examine  them  side  by  side  in  this  blue 
light.     The  fluorescence  will  then  be  more  noticeable. 

When  artificial  light  is  used  in  a  lantern  it  will  only 
be  necessary  to  place  the  colored  glass  in  front  of  the 
condenser,  as  if  to  project  a  picture  upon  it,  and 
otherwise  use  the  light  as  with  sunlight. 

Pictures  are  sometimes  made  of  fluorescent  material. 


124  THE  ART  OF  PROJECTING. 

The  outlines  of  flowers,  butterflies,  letters,  etc.,  are 
drawn  upon  paper  with  a  lead-pencil,  and  then  painted 
with  substances  that  exhibit  different  colors  by  fluores- 
cence. When  these  pictures  are  used  they  may  be 
pinned  to  the  screen  and  the  light  allowed  to  fall  upon 
them  as  before.  Examine  the  pictures  or  other  things 
by  light  transmitted  through  red,  yellow,  green,  blue, 
and  violet  glass.  The  kind  of  light  that  induces 
fluorescent  action  will  then  be  apparent. 

When  fluorescent  substances  are  to  be  examined  in 
the  light  of  a  solar  spectrum  they  may  be  made  to  pass 
through  it  from  the  red  towards  the  violet,  and  con- 
tinuing beyond  the  visible  part,  for  the  ultra-violet  rays 
are  capable  of  powerfully  exciting  fluorescence  in 
some  substances.  Stokes  found  this  invisible  spec- 
trum that  was  competent  to  induce  such  action  to  be 
as  much  as  three  or  four  times  the  length  of  the  visible 
spectrum. 

The  following  substances  manifest  fluorescent  ac- 
tion :  — 

Red  Fluorescence,       Chlorophyl, 

Orange      "  ** 

Yellow       "  Madder  mixed  with  Alum, 

Green        **^  Turmeric  Stramonium  and 

Night-shade, 
Brazil  wood,  Uranium  glass, 

Thallene, 
Blue  "  Quinine  —  horse  -  chestnut 

bark,  Petrolucene, 
Purple       "  Bichloranthracene. 

These  substances  are  generally  prepared  in  solutions 
or  decoctions  for  this  purpose. 

Chlorophyl  may  be  prepared  by  boiling  tea4eaves 
until  water  will  remove  nothing  more,  and  then  soak- 


LIGHT,  125 

iiig  them  in  hot  alcohol.  The  chlorophyl  will  thus  be 
extracted  and  the  tincture  is  ready  for  examination. 

A  few  pieces  of  the  hulls  of  horse-chestnuts,  or  of 
the  inner  bark  of  the  horse-chestnut  tree  digested  half 
an  hour  in  cold  water,  will  be  sufficient  for  this. 

Alcoholic  tinctures  of  madder,  stramonium,  night- 
shade, and  Brazil  wood  will  be  needed. 

A  grain  of  sulphate  of  quinine  may  be  put  into 
a  pint  of  pure  water  and  shaken  up  occasionally. 
This  substance  is  sparingly  soluble  in  water.  A 
little  tartaric  acid  may  be  added  to  the  water  with 
advantage :  the  fluorescence  will  be  more  strongly 
marked. 

A  good  method  of  exhibiting  this  is  to  have  a  rather 
large  glass  vessel  containing  pure  water  set  in  the  path 
of  the  violet  rays.  Pour  the  quinine  solution  into  it : 
opalescent  clouds  at  once  appear  to  form,  though  noth- 
ing is  precipitated. 

Thallene  and  anthracine  are  obtained  from  some  of 
the  products  of  the  distillation  of  petroleum  and  coal 
tar,  and  are  not  in  the  market. 

The  Aurora  tube  and  Geisler  tubes  when  lighted  by 
the  electric  spark  may  be  used  to  obtain  fluorescent 
effects.  With  the  former,  writing  and  drawings  made 
with  proper  solutions,  may  be  seen  when  such  markings 
would  be  entirely  invisible  in  common  white  light. 
Geisler  tubes  are  often  made  to  contain  some  pretty 
design  in  uranium  glass,  or  there  is  some  vessel  con- 
taining a  fluorescent  solution  surrounded  with  a  jacket 
filled  with  some  gas  which  gives  a  violet  light  like 
nitrogen. 

A  very  beautiful  effect  is  produced  by  exposing  a 
number  of  highly  fluorescent  media  to  the  flame  of 
sulphur  burning  in  oxygen  in  a  dark  room. 


126  THE  ART  OF  PROJECTING, 


DOUBLE    REFRACTION. 

A  piece  of  calc  spar  will  be  needed  to  show  this. 
Its  size  is  not  very  material,  though  the  thicker  it  is 
the  farther  apart  will  the  refracted  figures  be.  It 
should  have  smooth  faces,  but  the  natural  faces  are 
often  good  enough  to  permit  this  phenomenon  to  be 
projected. 


Fig*  9*. 


Make  a  hole  a  quarter  of  an  inch  in  diameter  through 
a  bit  of  cardboard  (unless  you  chance  to  have  a  dia- 
phragm with  holes  of  various  sizes)  and  place  it  at  the 
apertuie;  the  small  beam  of  light  which  comes 
through  it  should  be  directed  horizontally  upon  the 
screen.  Next  place  the  piece  of  spar  in  front  of  it, 
and  then  project  the  hole  with  an  object  lens  with  a 
foot  or  more  focus.  The  two  spots  will  appear  upon 
the  screen,  and  if  the  spar  be  rotated  the  one  spot 
will  revolve  about  the  other.  Instead  of  the  hole  in  a 
diaphragm,  it  will  do  as  well  to  make  a  black  spot  upon 
a  piece  of  glass  and  project  it  in  the  same  way. 

Either  side  of  the  spar  may  be  used  for  showing 
this  phenomenon. 

A  double-image  prism  may  also  be  used  with  still 
better  results  as  the  images  will  be  still  further  separated. 


LIGHT.  127 

POLARIZATION    OF    LIGHT. 

Plane-polarized  light  may  be  obtained  in  great 
quantity  by  using  for  a  reflector  in  the  porte  lumiere 
a  plate  of  glass  blackened  upon  its  back  surface. 
Choose  a  piece  of  good  window-glass,  without  bubbles 
or  striae,  and  paint  it  upon  one  side  with  lamp-black 
mixed  in  Japan  varnish.  It  will  be  best  to  lay  on  two 
or  three  coats  in  order  to  completely  cover  the  surface. 
Hold  it  between  the  eye  and  the  sun,  and  all  the 
uncovered  and  thin  places  can  be  seen ;  they  should 
receive  another  coat  of  paint.  This  painted  glass 
should  be  of  the  same  size  as  the  plane  mirror  in  the 
porte  luwiere^  upon  which  it  may  now  be  placed  and 
fastened  by  tying  about  them  both  a  thread  or  stretching 
a  ring  of  elastic  cord  over  them.  If  the  beam  of  light 
which  is  now  reflected  from  the  unpainted  surface  of 
this  glass  is  sent  through  a  double-convex  lens  and 
then  received  upon  the  screen,  it  will  be  seen  to  be 


Fig,  95. 

much  less  intense  than  the  beam  from  the  silvered 
mirror,  but  some  of  the  most  attractive  experiments  in 
the  whole  domain  of  physics  are  possible  with  this 
light. 

A  Nicol's  prism  n  will  be  necessary,  and  the  larger 
it  is  the  better,  but  very  good  effects  may  be  obtained 


I«8  THE  ART  OF  PROJECTING. 

with  such  small  ones  as  come  with  the  polarizing 
attachment  to  common  microscopes.  With  one  having 
a  face  three-quarters  of  an  inch  upon  a  side,  everything 
essential  can  be  shown  to  a  large  audience. 

1.  Place  the  prism  at  the  focus  of  the  lens  so  that 
all  the  light  will  pass  through  it.  Now,  if  the  prism  be 
rotated  upon  the  beam  as  an  axis,  the  disk  of  light  upon 
the  screen  will  decrease  in  brightness  until  it  is  nearly 
or  quite  invisible ;  and  if  the  prism  be  turned  still 
further  in  the  same  direction  the  light  will  reappear 
and  attain  its  maximum  brightness  when  the  prism 
has  been  turned  ninety  degrees  from  the  position 
where  the  light  disappeared. 

2.  Turn  the  prism  so  that  the  light  is  cut  off  from 
the  screen ;  and  then,  holding  it  in  that  position, 
slowly  introduce  a  thin  sheet  of  clear  mica  between 
the  lens  and  the  prism.  The  light  will  reappear  upon 
the  screen  from  that  transmitted  by  the  mica.  If  the 
mica  is  as  thin  as  the  fiftieth  of  an  inch,  or  less,  the 
light  may  be  colored  a  beautiful  blue  or  green  or  red. 
Turn  the  mica  round  in  its  own  plane,  and  these  colors 
will  appear  in  succession.  Let  the  prism  be  rotated 
while  the  mica  plate  is  held  still,  the  same  effects  will 
be  observed. 

3.  In  the  same  manner  experiments  with  thin  plates 
of  selenite  may  be  tried. 

4.  Bring  the  lens  forward  so  as  to  use  it  as  an  objec- 
tive, and  project  a  thin  piece  of  selenite  or  of  mica  with 
varying  thicknesses.  Hold  the  prism  in  the  focus  as 
before.  With  each  different  thickness  of  the  plates 
different  colors  will  be  transmitted  which  are  often 
ver)'  beautiful  indeed.  If  the  pieces  of  these  minerals 
are  not  more  than  an  inch  square,  a  larger  lens  may  be 
used  for  a  condenser,  and  then,  with  an  objective  of 


LIGHT. 


129 


four  or  five  inches  focus,  project  the  piece  in  the  same 
manner  precisely  as  you  would  with  the  solar  micro- 
scope. The  prism  must  be  held  in  the  focus  of  the 
objective  always. 

5.     Geometrical  designs  in  mica. 

Choose  a  thin  plate  of  mica 
that  is  clear,  and  three  inches 
or  more  square.  Hold  it  in  the 
polarized  light  and  see  if  it  pre- 
sents lively  colors ;  it  will  if  it 
is  thin  enough.  It  ought  not  to 
exceed  the  fiftieth  of  an  inch  in 
thickness  for  best  effect.  When  I 
the  tint  appears  uniform  over  ^^*  ^^' 

its  whole  surface,  as  it  will  if  its  thickness  is  uniform, 
it  may  have  drawn  upon  it  with  a  lead  pencil  such  a 
figure  as  the  accompanying  one,  and  then  trim  to 
the  edge  of  the  circle  with  scissors.  Afterwards,  with 
a  sharp  pen-knife  cut  about  one  fourth  of  the  way 
through  the  mica  on  all  the  lines ;  then  with  a  needle 
point  start  to  split  the  point  on  the  edge.  When  a 
thin  leaf  has  been  raised  a  little  between  two  points, 
carefully  move  the  needle  round  the  edge  so  as  to 
separate  the  same  thickness  all  around  the  circumfer- 
ence. Do  not  disturb  the  points  of  the  star  more  than 
at  the  extreme  point,  just  enough  to  keep  the  needle 
in  the  same  layer.  If  the  knife  has  cut  through  this 
layer  that  has  been  raised  at  the  edge,  the  parts  Oy  0,  o, 
can  be  removed,  leaving  the  six-pointed  star  a  little 
raised  above  the  surface  o,  0,  0. 

Examine  this  now  with  the  polarized  light,  and  the 

star  will  appear  to  be  of  one  color  and  the  cut-away 

parts  of  another.     If  the  interior  part  c  be  removed  it 

is  very  probable  that  that  part  will  exhibit  still  another 

9 


I30  THE  ART  OF  PROJECTING. 

color.  If  it  does  not,  it  is  because  the  part  removed 
had  the  same  thickness  as  one  of  the  others,  or  differ- 
ent from  it  by  a  wave  length.  Designs  of  any  kind 
that  fancy  may  dictate  may  be  thus  made  upon  sheets 
of  mica.  To  project  them  plainly  use  an  objective,  as 
in  4,  and  place  the  Nicols  prism  in  the  focus  of  it. 

Designs  in  selenite  are  still  handsomer,  and  figures 
of  birds,  butterflies,  flowers,  and  fruits  may  be  bought 
in  the  market.  Selenite  is  so  brittle  that  a  good  deal 
of  skill  is  needed  to  work  it,  and  it  would  be  tedious 
to  a  beginner.     Such  designs  had  better  be  purchased. 

6.  Unannealed  pieces  of  glass  when  they  have  a 
regular  form,  as  a  square,  a  triangle,  or  a  circle,  make 
good  objects  to  project  by  polarized  light.  They  are 
generally  a  quarter  of  an  inch  or  more  in  thickness,  and 
an  inch  or  two  in  diameter.  Pictures  of  their  appear- 
ance are  often  figured  in  works  on  physics.  Pieces 
of  thick  glass,  fragments  of  glass  vessels,  and  glass 
stoppers  of  bottles  often  show  double  refractive  power. 

7.  A  good  way  to  exhibit  the  development  of  this 
double  refraction  in  glass  is  to  take  a  piece  of  thick, 
plain  glass  and  stand  its  edge  upon  a  piece  of  iron, 
heated  to  redness,  projecting  the  whole  in  the  polar- 
ized beam  with  the  prism  in  its  place.  As  the  glass  is 
heated  and  strained,  colors  will  develop  upon  the  screen 
and  arrange  themselves  symmetrically,  depending 
wholly  upon  the  external  form  of  the  glass. 

8.  It  is  convenient  to  have  a  piece  of  glass  as  much 
as  a  quarter  of  an  inch  in  thickness  and  an  inch 
square  or  more,  that  is  annealed,  and  consequently 
gives  no  bands  or  colors.  If  this  is  strained  by  being 
pinched  in  a  hand-vice,  tufts  of  light  or  black  brushes 
will  be  seen  to  start  out  from  the  place  of  pressure  if 
the  whole  be  projected. 


LIGHT. 


131 


9.  A  bar  of  glass  half  an  inch  thick,  an  inch  broad, 
and  six  or  eight  inches  long,  may  be  gently  bent  with 
the  fingers  while  held  in  place  for  projection.  The 
strain  induces  double  refraction,  and  that  manifests 
itself  by  bands  of  light  or  dark,  or  color. 

All  of  these  should  have  their  outline  sharply  pro- 
jected by  an  objective  of  proper  focal  length. 

10.  A  small  crystal  of  Iceland  spar,  having  its  obtuse 
angles  ground  off  and  polished  so  as  to  present  a  sur- 
face as  much  as  a  quarter  of  an  inch  square,  will  pre- 
sent a  beautiful  series  of  rings  and  bars  when  projected. 


Vig.  97. 

It  will  not  be  necessary  to  use  an  objective,  but 
simply  to  put  both  crystal  and  prism  in  or  so  near  to 
the  focus  of  the  condenser  that  as  much  light  as  possi- 
ble may  be  transmitted  through  them.  When  in  place 
let  each  in  turn  be  rotated  upon  its  axis,  and  observe 
the  appearance  and  disappearance  of  the  light  and  dark 
bands.  At  a  distance  of  twenty  feet  from  the  prism  the 
outer  rings  should  be  about  four  feet  in  diameter. 

11.  A  crystal  of  rock  candy,  with  parallel  faces,  and 
not  more  than  the  twentieth  of  an  inch  in  thickness, 
will  present  another  system  of  rings  and  bands.  Pro- 
ject it  in  the  same  manner  as  the  spar  was  projected. 

12.  A  piece  of  a  quartz  crystal  cut  at  right  angles 
to  its  axis  will,  if  projected  in  the  same  way  as  the  last, 
exhibit  colors  upon  the  screen,  which  will  vary  as  the 


132 


THE  ART  OF  PROJECTING. 


prism  is  turned.  If  it  be  put  close  to  the  prism  there 
may  appear  a  system  of  concentric  circles  about  a  uni- 
form-colored field  in  the  centre.  The  colors  which 
this  central  field  assumes  when  the  analyzer  is  rotated 
are  often  superb. 

Spectacle  glasses  that  are  usually  called  Brazilian 
pebble  are  made  of  quartz,  and  such  will  exhibit 
brilliant  colors  by  projection  in  plane  polarized  light. 
This  serves  for  a  test  of  their  genuineness,  as  glass 
will  give  no  such  effect. 

1 3 .  The  system  of  bands  and  colored 
curves  seen  in  biaxial  crystals  is  not 
easy  to  project,  because  the  angles  at 
which  these  are  to  be  seen  are  so  great. 
With  some  crystals  of  potassium  nitrate 
it  is  possible  to  show  both  axes  at  a 
time  with  the  same  arrangement  as  was 
described  for  calc  spar.  A  clear  crys- 
tal about  the  quarter  of  an  inch  in 
diameter,  and  the  twentieth  of  an  inch 
thick,  may  answer  for  this.  Such  small 
crystals  are  usually  mounted  in  a  disk 
of  cork. 

Fig.  98  represents  the  double  system 
of  rings  and  brushes  seen  in  a  crystal 
of  nitrate  of  potash,  where  the  plane 
of  its  axes  coincides  with  the  plane  of 
the  polarizer;  and  Fig.  99  shows  the 
appearance  when  the  planes  are  slightly  inclined  to 
each  other. 

14.  There  are  many  minute  crystallizations,  such  as 
are  prepared  for  the  microscope,  that  make  fine  objects 
when  projected  in  polarized  light.  These  objects  may 
be  prepared  beforehand  ;  or  the  crystallization  with  the 


Fig.  99. 


LIGHT, 


133 


accompanying  development  of  polarization  properties 
may  be  projected.  It  will  be  simply  necessary  to  mag- 
nify the  object  by  using  a  lens  of  short  focus  as  in  the 
former  instruction  for  the  solar  microscope.  The  strip 
of  clear  glass,  holding  a  drop  of  a  saturated  solution 
of  the  substance,  the  objective,  and  the  Nicol's  prism 
being  put  near  the  focus  of  a  condenser  of  twelve  to 
eighteen  inches  focus,  that  the  specimen  may  be  lighted 
as  much  as  possible,  and  also  have  sufficient  light  trans' 
mitted. 


Fig,  100, 


The  following  list  of  salts  and  other  substances  wilj 
be  found  to  be  beautiful  objects  for  polarized  light :  — 


Alum, 

Oxalate  of  Ammonia, 

Borax, 

''    Lime, 

Carbonate  of  Lime, 

Oxalic  Acid, 

Carbonate  of  Soda, 

Picric  Acid, 

Chloride  of  Bariuir, 

Prussiate  of  Potash, 

"         "    Copper   and 

Salicine, 

Ammonia, 

Sulphate  of  Copper, 

Chloride  of  Sodium, 

u            \i          a 

Chlorate  of  Potash, 

Magnesia, 

Citric  Acid, 

Sulphate  of  Iron, 

Nitrate  of  Bismuth, 

"    Soda, 

*'         "    Copper, 

"    Zinc, 

"        ''    Potash, 

Sugar, 

and 


134 


THE  ART  OF  PROJECTING. 


Starch, 

Tartaric  Acid, 
Urea, 
Human  hair, 


Petals  of  flowers,  as  of  the 

Geranium, 
Scales  of  Fishes. 


Fig.  loi  represents 
the  appearance  of 
starch  grains  of  the 
potato,  as  seen  in 
common  light  with 
Fig.  101,  Fig.  102.         the  microscope,  and 

Fig.  I02,  the  same  seen  by  polarized  light 

The  following  method  of  preparing  double  salts  for 
examination  with  polarized  light  is  given  by  Mr.  Davies 
in  the  "  Quarterly  Journal  of  Microscopic  Science  "  :• — 
"  To  a  nearly  saturated  solution  of  sulphate  of  cop- 
per and  sulphate  of  magnesia  add  a  drop  on  the  glass 
slide,  and  dry  quickly.  To  effect  this,  heat  the  slide  so 
as  to  fuse  the  salts  in  its  water  of  crystallization,  and 
there  remains  an  amorphous  film  on  the  hot  glass. 
Put  the  slide  aside  and  allow  it  to  cool  slowly.  It  will 
gradually  absorb  a  certain  amount  of  moisture  from 
the  air,  and  begin  to  throw  out  crystals.  If  now 
placed  in  the  microscope,  numerous  points  will  be  seen 
to  start  out  here  and  there.  The  starting-points  may 
be  produced  at  pleasure  by  touching  a  film  with  a 
fine  needle  point  so  as  to  admit  of  a  slight  amount  of 
moisture  being  absorbed  by  the  mass  of  the  salt  " 

A  slide  of  salicine  crystals  makes  a  splendid  object 
for  such  projection,  and  should  be  in  every  collection. 
Make  a  saturated  solution  of  the  crystals  in  distilled 
water,  and  place  a  drop  carefully  upon  a  slide  that  has 
been  carefully  cleaned.  Evaporate  over  a  lamp  until 
it  is  dried  to  an  amorphous  mass.     Upon  cooling,  a 


LIGHT.  135 

number  of  circular  crystals  will  be  formed  with  radiat- 
ing forms  between  them.  These  circular  crystals  may 
be  made  larger  and  regularly  disposed  by  touching  the 
mass  with  a  fine  needle  point  when  crystallization 
begins.  Such  ones  will  form  about  each  point  touched. 
Magnify  such  objects  so  much  that  these  circular 
crystals  will  appear  a  foot  in  diameter  upon  the  screen. 
The  Nicol's  prism  will  show  each  one  with  four  arms 
that  will  turn  about  the  centre  of  the  crystal  when  the 
prism  is  rotated,  while  the  radiating  crystals  will  show 
as  red,  yellow,  or  purple  brushes  sweeping  over  the 
screen. 

By  inserting  a  sheet  of  transparent  mica  or  thin 
selenite  between  the  reflector  and  the  object,  a  colored 
field  will  appear  as  a  kind  of  background,  upon  which 
the  minute  crystals,  such  as  chlorate  of  potash  and 
oxalic  acid,  will  appear  more  highly  colored.  The  effect 
is  usually  to  heighten  the  color. 


THE   DOUBLE   IMAGE  PRISM. 


Fig*  103* 

With  a  large  lens  project  the  image  of  the  aperture 
upon  the  screen,  the  light  being  polarized  by  the  black- 
ened reflector.  At  the  focus  of  the  lens  place  the 
prism.     Two   images  o^  the  aperture  will  appear  and 


136  THE  ART  OF  PROJECTING, 

overlap  each  other.  Turn  the  prism  on  the  beam  as 
an  axis  j  the  images  will  turn  about  each  other. 

Place  a  thin  piece  of  mica  between  the  orifice  and 
the  lens.  The  two  disks  upon  the  screen  will  appear  in 
complementary  colors,  save  where  they  overlap,  which 
will  be  white.  Turn  the  prism  as  before  ;  the  colors  of 
the  two  disks  will  change,  always  being  complementary 
to  each  other. 

Again,  remove  the  reflector,  and  place  the  lens  close 
to  the  orifice.  Fix  the  prism  near  the  focus  so  that  a 
large  part  of  the  light  passes  through  it ;  and  then, 
with  lens  and  Nicol's  prism  near  it,  project  the  images 
of  objects  placed  close  to  the  double-image  prism.  In 
this  case  the  latter  acts  as  a  polarizer. 

When  large  Nicol's  prisms  can  be  had,  one  of  them 
may  be  substituted  for  the  reflector  upon  the  porte 
lumiere.     The  light  passing  through  it  will  be  polarized. 


Fig,  104, 

The  object  to  be  examined,  o,  may  be  placed  near  to  it 
in  front,  then  projected  with  any  convenient  lens,  in 
the  focus  of  which  place  the  other  Nicol's  prism.  This 
allows  a  large,  amount  of  light  to  be  used,  and  is  one 
method  in  use  with  lanterns.  The  only  hinderance  to 
the  use  of  these  larger  prisms  is  the  costliness  of  them. 
All  of  these  experiments  may  be  performed  with  a 
lantern,  with  one  of  the  more  powerful  lights.  The  usual 
method  of  polarizing  the  light  is  to  have   an  elbow  in 


LIGHT.  137 

front  of  the  condensers  that  carries  a  series  of  plain 
glass  plates  inclined  to  the  beam  so  that  it  meets  it  at 
the  polarizing  angle  of  glass.  Part  of  the  light  is  trans- 
mitted and  is  absorbed  by  a  piece  of  black  cloth.  The 
light  that  is  reflected  is  sufficiently  well-polarized  for 
all  purposes  of  demonstration  ;  and  such  a  beam  may 
be  treated  in  every  way  like  the  beam  from  the  port& 
lumiere  and  with  like  results. 

DIFFRACTION. 

Reflect  the  beam  from  the  parte  lumiere  through  a 
slit  like  one  for  showing  the  Fraunhofer  lines.  It 
ought  not  to  be  more  than  one  sixteenth  of  an  inch 
wide.  Receive  this  beam,  without  magnifying  it,  upon 
a  second  slit  in  a  screen  at  a  distance  of  four  or  five 
feet  from  the  first  slit  Make  the  room  as  dark  as 
possible,  and  then  hold  a  sheet  of  white  paper  behind 
the  second  slit  anywhere  from  a  few  inches  to  several 
feet.  Colored  fringes  will  appear  on  each  side  of  the 
central  line,  with  a  series  of  alternate  black  and  white 
bands  or  lines.  These  may  be  received  upon  a  screen 
twenty  feet  away,  when  they  should  have  a  united 
breadth  of  a  foot  or  more,  but  the  light  is  necessarily 
very  weak.     A  lens  does  not  improve  them  very  much. 

With  a  piece  of  perforated  paper  or  tin  or  lace,  or 
still  better,  with  an  eidotrope,  which  consists  of  two 
disks  of  perforated  tin  made  to  revolve  in  opposite  di- 
rections, like  the  chromatrope,  a  very  beautiful  exhi- 
bition of  the  phenomenon  of  diffraction  may  be  given 
in  the  following  way :  — 

Take  two  large,  short,  focus  lenses,  such  as  form  the 
condensers  in  Marcy's  sciopticon.  Place  one  close  to 
the  opening  to  XhQ  porte  lumiere,  as  shown  in  the  figure. 
The  second  one  may  be  put  so  far  in  front  of  the  other 


138  THE  ART  OF  PROJECTING. 

lens  that  the  beam  is  again  crossed  in  front  of  it,  and 
the  disk  upon  the  screen  is  of  the  desired  size,  six  or 
eight  feet  in  diameter.  Now  introduce  the  perforated 
paper  or  the  eidotrope  at  the  place  marked  e.     The 


Fig,  ion. 

screen  will  appear  covered  with  minute  spectra,  as 
each  hole  will  form  one  or  more  spectra ;  but  if  the 
paper  be  held  at  e,  between  the  lens  and  the  screen,  the 
projection  becomes  very  gaudy  and  symmetrical.  If  it 
be  the  eidotrope,  turn  it  while  held  in  that  place,  and 
the  colors  will  change  and  will  rival  the  colors  pro- 
duced by  polarized  light.  Try  the  effect  of  a  comb, 
of  wire  gauze,  of  the  fingers,  or  other  objects.  Very 
curious  and  interesting  appearances  will  appear  upon 
the  screen. 

If  one  has  a  j^iece  of  glass  finely  ruled  with  a  dia- 
mond, it  may  be  projected  as  is  any  object  with  the 
porte  himicre,  and  the  diffraction  spectra  will  appear 
upon  the  screen.  With  plenty  of  light  for  the  projec- 
tion, and  with  the  room  otherwise  well  darkened,  a 
number  of  the  Fraunhofer  lines  may  be  seen  in  these 
spectra. 

Again,  let  a  concentrated  solution  of  alum  or  cam- 
phor be  poured  upon  a  glass  plate,  and  allowed  to  dry 
rapidly,  so  as  to  cover  it  with  a  crust.  Put  it  in  the 
focus  of  a  lens  with  a  short  focus,   and  a  series   of 


LIGHT. 


139 


halos  will  be  seen  by  placing  a  small  screen  but  a  foot 
or  two  from  the  glass.  Fine  rulings  upon  blackened 
glass  will  in  the  same  place  give  fine  colors.  These 
rulings  may  be  as  coarse  as  fifty  to  the  inch ;  the  finer 
rulings  will  answer  better.  The  rapidly-diverging  rays 
necessitate  the  placing  of  the  screen  close  to  the  plate, 
else  the  colors  will  be  too  faint. 


PERSISTENCE   OF   VTSTON.  — THE   STROBOSCOPE. 

Let  a  disk  a  foot 
in  diameter  be  cut 
out  of  any   conve-! 
nient  material, — tin, 
copper,    zinc,    o  ri 
pasteboard.      Near 
the    periphery    cut 
out    a    number     of| 
holes  at  equal  dis-i 
tances  apart,  —  ten 
or    twelve    will    be 
enough.    They  may 

be  cut  half  an  inch  pig,  jo«. 

is  diameter.  This  disk  is  to  be  put  upon  a  rotator  like 
the  one  used  to  show  the  Newton's  disk,  and  may  now 
be  placed  so  that  the  focus  of  the  condenser  with  the 
porie  lumiere  will  be  in  the  holes  as  the  disk  revolves, 
as  in  Fig.  106.  This  permits  the  light  to  pass  to  the 
screen  only  when  the  holes  are  at  the  focus,  at  which 
time  a  powerful  beam  will  pass  and  is  immediately  cut 
off.  With  such  a  fixture  a  very  great  number  of  amus- 
ing and  instructive  experiments  may  be  made. 

I.  While  one  person  turns  the  stroboscopic  disk 
let  another  one  stand  in  front  of  the  screen  and  swing 
his  arms,  or  move  his  body  rapidly  sideways,  or  make 


I40  THE  ART  OF  PROJECTING, 

low  courtesies.  To  spectators  he  will  appear  to  have 
a  dozen  arms  or  bodies.  There  will  also  be  as  many 
shadows  upon  the  screen. 

2.     Make  a  wheel  to  turn  in  front  of  the  screen,  the 


Fig,  107. 

larger  the  wheel  the  better.  A  buggy  wheel  or  old 
fashioned  spinning-wheel  make  good  objects.  Let  both 
disk  and  wheel  be  turned  at  the  same  time.  The  ap- 
pearance of  the  wheel  will  depend  upon  its  velocity. 
It  may  be  made  to  appear  as  if  standing  still  or  moving 
slowly  forward  or  backward,  or  as  if  it  had  a  multitude 
of  spokes. 

3.  AVhile  the  wheel  is  turning  a  little  in  front  of  the 
screen,  look  through  the  wheel  at  the  shadow  of  it. 
Some  remarkable  curved  lines  will  appear  to  group 
themselves  about  the  axles  of  the  wheel  and  its  shadow. 

4.  If  two  wheels  of  the  same  size  are  made  to  turn 
one  in  front  of  the  other  while  they  are  in  this  inter- 
mittent -light,  curious  curves  and  fixed  straight,  light 
or  darTv  lines  and  mixed,  changing  paths  can  be  seen, 
according  to  the  position  the  spectator  has  with  refer- 
ence to  the  wheels. 


LIGHT.  1-41 

5.  If  a  small  wheel  but  two  or  three  or  four  inches 
in  diameter,  and  toothed  like  a  cog-wheel  in  a  clock,  be 
placed  within  a  foot  or  two  of  the  disk,  and  so  that  its 
shadow  will  fall  upon  the  screen,  its  shadow  will  not 
only  be  much  magnified,  but  tlie  motions  of  the  wheel 
will  appear  as  with  the  lai^er  one,  number  i . 

6.  Let  large  disks  three  or  four  feet  in  diameter  be 
made,  having  various  symmetrical  figures  painted  upon 
them.  When  the  disks  are  revolved,  many  curious 
motions  may  be  simulated  :  as  of  a  girl  jumping  rope, 
a  man  sawing  or  chopping  wood,  boys  playing  leap- 
frog, a  man  opening  and  shutting  his  eyes  and  mouth, 
wind-mill  with  sails  turning,  etc.  Such  designs  gen- 
erally come  with  the  toy  called  the  thaumatrope, 
made  to  look  through  into  a  mirror  while  turning. 
These  may  be  copied  upon  large  sheets  of  pasteboard 
and  rotated  in  any  convenient  way.  The  turning-table 
may  be  made  to  answer  for  this. 

7.  Another  good  and  very  pretty  application  of 
this  is  to  have  a  large  star  with  five  or  six  points  made, 
and  the  alternate  points  colored  with  different  tints, 
as  red  and  blue.  When  such  a  disk  is  revolved  in  this 
intermittent  light  it  may  appear  to  stand  very  still,  or 
to  slowly  revolve  forward  or  backward,  while  its  points 
may  be  doubled  or  tripled  or  quadrupled,  and  its  colors 
will  apparently  overlap  and  give  the  tints  proper  to  the 
mixture. 

8.  Such  pictures  as  are  sold  with  the  thaumatrope 
may  be  fastened  to  the  front  of  the  disk  containing 
the  holes  through  which  the  light  passes,  as  is  repre- 
sented in  Fig.  107,  and  after  the  light  has  passed  through 
the  disk,  it  may  be  reflected  upon  its  face  by  a  small 
mirror,  m  (Fig;.  108),  and  can  thus  be  seen  very  well  if 
the  light  be  strong.     When  used  in  thfe  way  the  disk 


142 


THE   ART  OF  TROJECTIATG. 


may  be  made  very  much  larger,  as  much  as  two  or 
three  feet  in  diameter,  and  the  number  of  holes  in- 
creased. By 
removing  the 
mirror  m  a 
[little  farther 
away  the  beam 
can  be  reflect- 
ed so  as  to  cov- 
ler  the  whole 
Fig,  108.  faceof  the  disk. 

A  small  toy  steam-engine,  such  as  may  be  bought 
for  a  dollar  or  two,  may  have  a  light  paper  disk  fitted 
for  it  to  turn,  but  if  sunlight  be  used,  care  must  be 
taken  lest  it  take  fire  in  the  focus  of  the  sun's  rays. 

An  oxyhydrogen  lantern  may  be  used  for  such  work. 
The  objective  will  need  to  be  removed,  and  the  perfo- 
rated disk  placed  so  that  the  most  of  the  light  goes 
through  the  holes  when  they  are  in  position,  and  the 
unused  light  cut  off  from  entering  the  room  by  black 
cloth  or  some  other  provision.  Otherwise  it  will  be 
used  just  as  with  sunlight. 


THE    CHROMATROPE. 


This  instrument  consists  of  two  disks  of  glass  so 
mounted  that  they  may  be  rotated  in  opposite  direc- 
tions. Various  designs  are  painted  upon  the  disks, 
and  fine  effects  may  be  obtained  by  projecting  them  in 
the  ordinary  way  with  the  lantern  or  the  J>or^e  lumiere. 
If  instead  of  using  disks  of  glass,  disks  are  made  of 
wire  gauze,  perforated  tin,  or  paper  or  lace,  very  curious 
interference  figures  are  produced,  and  this  form  is 
called  the  eidotrope. 


LIGHT. 


143 


TRe  accompanying  figure  represents  a  chromatrope 
with  an  arrange-  .„.         . 

ment  for  quickly   ^^___ j 

replacing  one 
disk  by  another 
of  different  pat- 
tern. Rotation 
is  given  by  fric- 
tion pulleys. 
With  this  form 
there  is  a  disk 
with  the  so-called 
seven  primary 
colors  to  illus- 
trate     Newton's        

theory  of  colors,         i  5  3  4 

one  to  illustrate  Brewster's  theory,  two  to  illustrate 
Young's  theory,  and  a  chameleon  top,  designed  by 
President  Morton,  of  Stevens  Institute,  Hoboken. 

The  effects  with  all  the  forms  of  chromatropes  are 
due  to  persistence  of  vision. 

Interesting  subjective  effects  may  be  observed  by 
projecting  in  the  ordinary  way  bits  of  colored  glass  an 
inch  or  two  square,  so  as  to  have  upon  the  screen  a  large 
patch  of  color  with  a  boundary  of  white  light.  The  eyes 
must  be  fixed  attentively  upon  the  colored  patch  for 
about  half  a  minute,  when  the  colored  piece  must  be 
quickly  removed,  the  eyes  to  be  kept  meanwhile  upon 
the  screen.  To  prevent  the  eyes  from  unconsciously 
wandering  while  lacking,  it  will  be  found  advisable  to 
pin  a  large  black  button  or  a  piece  of  black  paper  to 
the  screen  in  the  middle  of  the  disk.  This  is  to  be 
kept  in  the  centre  of  vision.  The  effects  observed  will 
of  course  depend  upon  the  color  upon  the  screen,  and 


144  THE  ART  OF  PROJECTING. 

the  sensitiveness  of  the  eyes  for  various  colors.  Gen- 
erally, after  looking  steadily  at  a  given  color,  and  the 
disk  is  made  suddenly  white,  the  outline  of  the  colored 
part  will  be  seen  in  a  color  complementary  to  the  one 
looked  at  first.  Thus,  if  a  square  red  glass  should  be 
projected  the  residual  image  would  be  a  square  green 
one.  If  a  blue  one  was  projected  its  complementary 
image  would  be  orange,  and  so  on.  A  great  variety  of 
su:h  effects  are  obtainable  with  various  colored  pieces 
of  glass,  or  of  films  of  gelatine^  by  projecting  them 
singly,  in  juxtaposition,  or  superposed. 

Let  disks  of  white  cardboard  a  foot  or  two  in  diame- 
ter have  partial  sectors  painted  black,  with  India  ink, 
so  that  the  white  and  black  parts  alternate  four  or  five 
times  in  the  circumference.  This  is  to  be  rotated  while 
a  powerful  beam  of  light  falls  upon  it.  The  persist- 
ence of  some  of  the  elements  of  white  light  being 
greater  than  of  others,  the  disk  will  appear  of  various 
colors  ;  purple,  green,  and  yellow  being  generally  well 
developed. 

HEAT.  AIR    THERMOMETER. 

A  bulb  blown  upon  one  end  of  a  small  glass  tube, 
five  or  six  inches  long,  answers  for  this  experiment.  A 
drop  of  colored  water  can  be  made  to  enter  the  tube 
by  first  heating  the  bulb  a  little  by  holding  it  in  the 
fingers  with  the  open  end  of  the  tube  a  little  below  the 
surface  of  the  water.  A  bubble  or  two  of  air  will  be 
expelled,  and  the  fingers  may  be  removed  from  the 
bulb.  As  it  cools  a  drop  will  be  driven  into  the  tube, 
and  with  a  little  painstaking  it  can  be  brought  to  any 
required  place  by  cooling  or  heating  the  bulb.  These 
movements  can  be  shown  with  the  pOT^e  lumiere  and  a 
single  lens,  as  shown  in  Fig.   17,  or  it  can  be  put  in 


HEAT. 


145 


front  of  the  condenser  of  the  lantern.  A  touch  of  the 
finger  will  heat  the  bulb  sufficient  to  cause  the  drop  to 
rise  in  the  tube,  and  it  may  be  made  to  descend  by 
simply  blowing  upon  the  bulb,  or  by  dropping  a  little 
water  or  ether  upon  it. 

Many  of  the  pieces  of  apparatus  for  illustrating  the 
expansion  of  metals  by  heat  are  so  small  that  they  may 
be  readily  projected.  Thus  Gravesand's  Ring,  Pyrom- 
eters, etc.  The  latter  may  have  a  small  bit  of  mirror 
fastened  to  the  end  of  the  index,  and  the  light  so 
arranged  that  as  the  index  rises,  the  beam  will  move 
upward.  A  rise  in  temperature  of  only  a  few  degrees 
can  be  then  shown,  and  the  alcohol  flame  may  be  dis- 
pensed with  ;  the  warmth  of  the  hand  or  a  little  hot 
water  answering  the  purpose. 

FORMATION   OF   CLOUDS. 

The  condensation  of  liquid  in  the  form  of  vapor  mto 
minute  globules  and  in  the  production  of  a  shower  of 
rain  may  be  very  well  illustrated  and  projected  for 
class  purposes  in  the  following  manner :  — 

Place  about  an  ounce  of  Canada  balsam  in  a  Flor- 
ence flask  and  make  it  boil.  At  the  top  of  the  flask 
clouds  of  globules  of  turpentine  will  be  seen  hovering 
about,  altering  in  shape  very  much  like  sky  clouds,  and 
the  globules  are  large  enough  to  be  visible  by  the  naked 
eye.  If  a  cold  glass  rod  be  gradually  introduced  into 
the  flask  these  clouds  may  be  made  to  descend  in 
showers.  Lawson  Tait  in  Nature. 

Another :  Take  a  flask  of  one  or  two  litres  capacity  j 
rinse  it  out  with  distilled  water,  and  attach  to  the  neck 
a  cork  and  glass  tube  of  about  twenty  or  thirty  centi- 
metres length.     Place  the  glass  tube  in  the  mouth  and 

10 


146 


THE  ART  OF  PROJECTING. 


exhaust,  when  a  dense  cloud  will  be  formed ;  then  on 
blowing  into  the  flask  the  cloud  disappears.  The  cloud 
may  be  produced  and  dissolved  as  often  as  wished, 
and  if  a  beam  from  the  oxyhydrogen  light  be  sent 
through  the  flask  the  experiment  becomes  very  effec- 
tive. C  y.  Woodward  in  Nature, 


MAXIMUM    DENSITY    OF    WATER. 

Take  a  small  test-tube,  not  more  than 
two  or  three  inches  long  and  half  an  inch 
in  diameter,  and  through  a  tight-fitting 
cork  thrust  a  small  glass  tube  about  three 
inches  long,  allowing  it  to  project  as 
much  as  two  inches.  Fill  the  test-tube 
with  water  at  about  4°  centigrade  and 
cork  it  tight,  so  that  the  water  will  rise  in 
the  glass  tube.  See  that  there  are  no  air 
bubbles  beneath  the  cork.  Mark  the 
height  of  the  water  in  the  small  tube  by 
tying  a  thread  about  it.  Project  the 
whole  with  a  lantern  or  with  the  p07'te 
lumiere.  Now,  if  a  small  vessel  contain- 
Fig*  109.  ing  hot  water  be  brought  up  under  the 
test-tube  so  that  the  latter  dips  in  it,  the  expansion  of 
the  water  will  be  indicated  by  the  rise  of  the  water  in 
the  tube,  and  the  latter  will  overflow  if  it  be  sufficiently 
heated.  Now,  bring  up  under  it  in  the  same  way  a 
freezing  mixture  of  ice  and  salt,  or  a  mixture  of  equal 
parts  of  cold  water  and  nitrate  of  ammonium.  The 
water  will  contract  in  volume  to  its  minimum,  which 
should  be  indicated  by  the  thread;  then  it  will  again 
expand  until  it  freezes,  the  expansion  again  causing  the 
water  in  the  tube  to  overflow.  The  freezing  mixture 
should  be  stirred  constantly  to  hasten  the  work. 


HEAT,  147 

THE   GALVANOMETER. 

As  many  of  the  experiments  in  heat  require  the 
thermo-pile  and  galvanometer,  the  latter  is  treated  of 
in  this  place  rather  than  with  electrical  experiments. 

In  the  "American  Journal  of  Science,"  Vols.  II,  III, 
V,  IX  and  X,  are  given  several  ingenious  arrangements 
for  projecting  the  movements  of  a  galvanometer  needle, 
and  if  one  desires  to  do  extremely  accurate  work  be- 
fore an  audience  he  will  do  well  to  obtain  some  one  of 
these  forms.  If,  however,  it  is  desirable  only  to  ex- 
hibit qualitatively  and  with  no  great  degree  of  precision 
the  relation  of  heat  to  electricity,  or  the  law  of  the 
galvanometer,  etc.,  the  following  method  will  be  found 
to  answer,  with  the  advantage  of  being  extemporized 
in  a  few  minutes :  Make  2iflat  coil  about  an  inch  square, 
of  rather  fine-covered  copper  wire  having  the  ends 
of  the  wire  a  foot  or  more  in  length.  Upon  one  side 
of  this  coil  stick  a  bit  of  beeswax  as  large  as  a  small 
marble,  and  through  both  wax  and  coil  thrust  half  of  a 
fine  cambric  needle.  Press  the  wax  down  upon  the 
middle  of  a  piece  of  glass  four  or  five  inches  square, 
and  then  holding  the  plate  horizontal,  suspend  upon 
the  needle  point  a  small  compass  needle  an  inch  or  two 
long.  This  is  now  ready  to  place  upon  the  upper  con- 
denser c  (Fig.  27)  of  the  vertical  attachment  and 
then  be  projected.  If  a  current  from  a  battery  or  a 
thermo-pile  be  sent  through  the  coil,  the  needle  will  be 
deflected.  The  needle  will  of  course  point  towards 
the  north,  and  that  place  will  easily  be  noted  upon  the 
screen  as  zero.  A  small  magnet  brought  into  the 
neighborhood  will  serve  to  bring  the  north  pole  of  the 
needle  to  any  required  place.  If  a  circle  with  in- 
scribed degrees  should  be  drawn  upon  the  glass  by 


148  THE  ART  OF  PROJECTING. 

either  of  the  methods  described  upon  pages  31  or  32, 
the  movement  of  the  needle  can  be  noted  in  degrees. 
If  the  needle  is  too  short  to  reach  the  numbers  upon 
the  glass,  it  can  have  a  fine  straight  bristle  made  fast 
to  its  ends  with  a  little  mucilage. 

With  the  thermo-pile  connected  with  the  galvanom- 
eter, the  sensitiveness  of  the  former  may  be  shown  by 
presenting  the  hand  to  one  face  of  it,  or  it  may  be 
breathed  upon  or  blown  upon  with  a  common  hand 
bellows.  Let  fall  a  drop  of  water,  of  ether,  and  of 
alcohol  upon  the  face.     The  evaporation  cools  it. 

The  heat  generated  by  percussion  may  be  exhibited 
by  providing  a  leaden  bullet  which  should  have  at  first 
the  same  temperature  as  the  thermo-pile,  which  may  be 
known  by  putting  it  upon  the  pile,  handling  it  with  a 
pair  of  small  tongs.  It  should  not  move  the  needle. 
Then  strike  it  once  with  a  hammer  so  as  to  indent  it 
considerably,  and  with  the  tongs  quickly  put  it  again 
upon  the  face  of  the  pile.  It  will  indicate  a  higher 
temperature. 

The  heat  generated  by  friction  may  be  shown  by 
rubbing  a  stick  upon  the  floor  and  then  bringing  it  to 
the  pile  as  in  the  other  case. 

See  Tyndall's  work  on  Heat  for  a  method  of  show- 
ing heat  from  the  crystallization  of  sodium  sulphate. 
The  same  thing  may  be  shown  with  the  air  thermom- 
eter sunk  into  the  solution,  which  may  be  projected 
with  lantern  or  porte  lutniere  by  preparing  the  solution 
in  a  beaker,  fixing  the  air  thermometer  in  it  with  a  drop 
of  colored  water  in  it,  and  projecting  the  whole  upon 
the  screen  by  means  of  a  large  lens.  The  crystalliza- 
tion itself  will  be  seen,  as  well  as  the  manifested  heat, 
when  it  reaches  the  bulb  of  the  thermometer. 

Mix  in  a  test-tube  resting  upon  the  face  of  the  ther- 


HEAT. 


49 


mo-pile,  a  few  drops  of  water  and  sulphuric  acid  about 
equal  parts :  the  heat  evolved  will  illustrate  the  origin 
of  heat  from  chemical  reaction. 

A  few  crystals  of  nitrate  of  ammonium  in  a  test-tube 
may  have  an  equal  bulk  of  water  poured  upon  them ; 
the  cold  produced  is  from  the  absorption  of  heat  dur- 
ing liquefaction. 

Interpose  between  the  source  of  heat  and  the  ther- 
mo-pile  various  things,  such  as  rock-salt,  a  solution  of 
iodine  in  bisulphide  of  carbon,  glass,  crystals  of  various 
kinds,  tubes  filled  with  gases  and  vapors  of  various 
sorts.  Also,  project  a  solar  spectrum  with  a  part  of 
the  same  beam  that  projects  the  galvanometer  by  the 
method  described  upon  page  112.  Move  the  thermo- 
pile through  the  various  colors,  and  note  the  degree 
indicated  by  the  galvanometer,  particularly  beyond  the 
red  end  of  the  spectrum.  The  thermo-pile  should  be 
placed  where  the  Fraunhofer  lines  are  seen  best  upon 
a  small  screen  placed  temporarily  to  receive  it. 

Many  experiments  on  this  subject  will  be  found  in 
Tyndall's  work  on  Heat,  which  one  will  find  himself 
able  to  repeat  with  satisfaction. 


CALORESCENCE. 

Let  the  light  from  theporte  lumierCy  or  from  the  elec- 
tric or  lime  light,  be  sent  through  a  vessel  containing 
bisulphide  of  carbon  in  which  some  iodine  has  been 
dissolved :  the  solution  will  be  jet  black  and  will  stop 
every  light  ray,  but  will  permit  the  rays  of  greater  wave 
length  to  freely  traverse  it.  A  lens  may  now  be  inter- 
posed and  the  obscure  rays  treated  in  every  way  like 
luminous  rays.  With  a  very  powerful  beam  platinum 
foil  may  be  raised  to  incandescence  in  the  focus  of  the 


ISO  THE  ART  OF  PROJECTING. 

lens,  and  with  a  less  powerful  one  pieces  of  wood  and 
paper  may  be  ignited. 

A  transparent  solution  of  common  alum  is  opaque 
to  the  same  rays  that  are  so  easily  transmitted  by  the 
iodine  solution. 

A  test-tube  filled  with  water  placed  at  the  focus  of 
the  obscure  rays  in  a  minute  or  two  may  be  made  to 
boil ;  an  air  thermometer  will  scarcely  be  affected  at 
that  place. 

MAGNETISM. 

With  the  vertical  attachment  to  the  lantern  the 
phenomena  of   magnetism   are  best  shown. 

1.  Have  two  or  three  small  magnetic  needles 
mounted  upon  needle  points  thrust  through  pieces  of 
cork,  so  as  to  turn  freely.  Place  one  upon  the  upper 
face  of  the  condenser  to  the  vertical  attachment,  and 
project  it  sharply  upon  the  screen.  A  piece  of  iron  or 
another  magnet  brought  into  its  neighborhood  will 
disturb  it,  and  every  motion  will  be  plainly  noticeable 
as  well  as  the  direction  of  the  exciting  body. 

2.  Place  two  of  these  needles  near  to  each  other, 
but  not  so  near  as  to  touch,  and  give  to  one  of  them  a 
twirl  so  that  it  revolves  upon  its  support.  It  will  soon 
set  the  other  revolving  and  it  may  be  stopped  itself 
after  setting  the  second  one  going,  and  afterward  be 
again  started  while  the  other  one  stops. 

3.  Place  a  third,  quite  small  one  not  more  than 
half  an  inch  long  in  the  neighborhood  of  the  other 
two,  and  again  set  the  one  whirling. 

4.  The  magnetic  phantom. 

Lay  a  small  magnet  an  inch  or  two  long  upon  the 
upper  condenser  ;  and  upon  the  magnet  lay  a  piece  of 
clear  glass  three  or  four  inches  square.  Project  the 
magnet,  and  then  scatter  from  a  small  sieve,  or  gently 


MAGNETISM. 


151 


with  the  thumb  and  finger,  fine  iron  filings  upon  the 
glass.  The  filings  will  arrange  themselves  in  the 
familiar  lines  called  the  magnetic  phantom,  and  the 
whole  being  magnified  to  ten  feet  or  more  in  diameter 
makes  a  very  striking  pictyre. 

5.  The  elongation  of  an  iron  rod  when  strongly 
magnetized,  may  be  shown  by  placing  a  small  helix 
around  the  iron  rod  of  the  common  pyrometer  made 
for  showing  the  longitudinal  expansion  of  a  rod  by 
heat.  To  the  end  of  the  index  finger  that  sweeps  over 
the  quadrant  affix  a  small  bit  of  plane  mirror  not  more 
than  one  fourth  of  an  inch  square.  So  adjust  the  light 
to  this  small  mirror  that  the  reflection  from  the  latter 
will  fall  upon  the  most  distant  part  of  the  room  ;  the 
farther  away  the  better.  When  the  current  of  elec- 
tricity is  sent  through  the  helix  the  rod  will  be  slightly 
elongated,  but  the  slight  tilting  of  the  mirror  may 
become  a  displacement  of  two  or  three  inches  at  a 
distance  of  thirty  feet. 


DIAMAGNETISM. 

The  electro-magnet  'for  demonstrating  diamagnetic 
phenomena  need  not  be  over  three  or  four  inches  in 
length,  and  the  poles  an  inch  apart. 
Objects  to  be  tested  may  be  suspen- 
ded by  a  thread  between  the  poles, 
and  the  whole  projected  either  in  a 
beam  of  parallel  rays  or  in  front  of 
the  focus  of  a  lens.  In  the  latter 
case  the  whole  will  be  seen  in  pro- 
file, but  perfectly  distinct.  The  fol- 
lowing experiments  may  be  projected 
with  such  a  magnet  if  a  battery  of  Tig.  110, 
three  or  four  cells  be  used :  — 


152  THE  ART  OF  PROJECTING. 

1.  Suspend  oblong  pieces  of  various  metals  half  an 
inch  in  length,  and  note  whether  they  set  themselves 
equatorially  or  axial ly  between  the  poles.  Iron, 
nickel,  platinum,  bismuth,  antimony,  zinc,  tin,  lead, 
silver,  copper,  alum,  glass,  sulphur,  sugar,  bread,  paper, 
charcoal,  are  good  substances  to  experiment  with. 

2.  Suspend  a  cube  of  copper  between  the  poles, 
and  twist  the  thread  so  that  the  copper  will  rotate 
rapidly  by  torsion.  It  will  quickly  be  brought  to  rest 
when  the  current  is  made  to  pass. 

3.  Fill  small  very  thin  tubes  with  liquids,  and  sus- 
pend them  in  the  same  manner.  Try  solutions  of  iron, 
cobalt,  water,  alcohol,  turpentine,  and  salt. 

4.  Place  the  magnet  upon  the  upper  condenser  of 
the  vertical  attachment,  and  upon  its  poles  place  a 
watch-glass  containing  a  little  water  or  sulphuric  acid ; 
project  the  water  in  the  watch-glass,  and  notice  the 
distribution  of  light  upon  the  image  of  the  water. 
Now  complete  the  circuit.  The  water  will  change  its 
form  slightly  and  the  light  will  be  differently  refracted, 
thus  making  it  quite  visible.  Salts  of  iron  or  nickel 
will  scatter  the  light  like  a  concave  lens. 

5.  Hold  the  flame  of  a  candle  between  the  poles. 

6.  Blow  small  soap  bubbles  with  oxygen  and  with 
illuminating  gas,  and  hold  them  as  close  to  the  poles  as 
possible  or  drop  them  so  they  will  rest  upon  both. 

7.  Heat  a  coin  and  place  it  just  beneath  the  poles, 
and  then  drop  a  piece  of  iodine  upon  the  coin.  The 
heat  will  volatilize  the  iodine,  and  the  purple  vapor  will 
be  repulsed. 

ELECTRICITY. 

Most  of  the  experiments  in  electricity  which  can  be 
showp  by  projection  require  the  use  of  the  galvanom- 
eter, such  for  instance  as  give  evidence  of  the  existence 


RLECTRICnV,  153 

of  electrical  currents,  their  direction  and  strength. 
These  will  only  need  the  arrangement  already  described 
under  the  head  Galvanometer.  For  other  experiments, 
such  as  that  of  the  electric  light,  there  will  be  needed 
some  one  of  the  many  fixtures  for  holding  the  carbons 
to  be  ignited.  .  If  this  can  be  put  into  a  lantern  the 
carbons  may  be  projected  at  once  upon  the  screen  by 
removing  the  objective  and  drawing  the  carbons  back 
until  the  image  appears  plainly  upon  the  screen.  This 
image  will  be  made  much  sharper  by  putting  a  dia- 
phragm with  about  an  inch  aperture  over  the  conden- 
sers, which  in  this  case  serves  for  an  objective. 

For  the  projection  of  spectra  precisely  the  same 
conditions  need  to  be  observed  as  for  the  lime  light:  — 
Some  regulator  in  the  lantern,  a  slit  in  the  focus  of  the 
condensers,  an  objective  to  project  the  slit  and  the 
prism  in  the  focus  in  front  of  the  objective.  The  spec- 
trum of  metals  is  easy  with  this  arrangement.  Make 
a  small  cavity  'n  the  end  of  the  lower  carbon  stick, 
and  place  a  small  bit  of  the  metal  whose  spectrum  is 
wanted  in  it ;  then  bring  down  the  upper  carbon  upon 
it  so  as  to  complete  the  circuit  and  then  raise  it  a  little, 
the  metal  will  be  at  once  fused  and  volatilized,  emit- 
ting its  characteristic  light,  which  will  appear  upon  the 
screen  as  bright  bands.  Silver,  copper,  zinc,  iron,  and 
mercury  give  good  spectra  among  the  more  common 
elements. 

For  the  successful  working  of  this  method  of  spec- 
trum analysis,  not  less  than  forty  cells  will  be  needed, 
and  fifty  are  decidedly  better  than  forty. 

DECOMPOSITION   OF   WATER. 

This  is  effected  by  sending  a  current  of  electricity 
from  three  or  four  cells  through  water  that  has  been 


154 


THE  ART  OF  PROJECTING. 


slightly  acidulated  by  the  addition  of  a  little  sulphuric 
acid.  The  terminals  of  the  wires  in  the  water  are 
usually  made  of  strips  of  platinum  to  prevent  other 
chemical  reactions  from  taking  place.  For  projection, 
an  excellent  way  is  to  insert  two  test-tubes  filled  with 
the  acidulated  water,  and  introduce  them  into  the  tank 
already  described,  having  previously  fixed  the  two 
platinum   terminals   through    the    rubber    bottom   as 


Fig,  lit. 

shown  in  Fig.  iii.  When  the  current  is  sent  through 
these  wires  the  bubbles  will  rise  rapidly  and  soon  fill 
the  hydrogen  tube.  This  tank  is  of  course  to  be  pro- 
jected in  the  ordinary  way,  either  with  lantern  or  parte 
lumiere^  in  which  case  the  bubbles  will  appear  very 
large  and  the  water  will  appear  to  be  in  great  commo- 
tion. 

In  place  of  water  fill  the  tank  with  a  solution  of 
acetate  of  lead,  and  without  the  test-tubes  project  the 
tank  and  make  connection  with  the  battery  of  two  or 
three  cells  as  before  :  the  crystallization  of  the  lead  will 
at  once  begin  and  rapidly  grow  upon  one  of  the  termi- 
nals j  reverse  the  current,  and  the  formed  crystals  will 


ELECTRICITY. 


155 


dissolve  while  others  will  grow  upon  the  other  terminal. 
The  same  thing  can  be  done  still  better  by  filling  the 
horizontal  tank  for  the  vertical  attachment  with  the 
solution  of  lead  acetate,  and  then  bending  a  piece  of 
platinum  wire  or  of  tin  wire  around  the  interior  of  the 
tank.  Then,  on  inserting  another  wire  at  the  centre  of 
the  solution,  and  making  connection  with  two  or  three 
cells  so  as  to  make  the  centre  wire  the  negative  and  the 
hoop  the  positive  pole,  a  beautiful  growth  of  metallic 
crystals  will  shoot  out  from  the  centre  and  spread  out 
over  the  entire  field.  In  place  of  the  solution  of  lead 
use  a  strong  solution  of  the  bichloride  of  tin,  using  a 
tin  hoop  in  the  solution.  Crystals  of  tin  will  shoot  out 
and  appear  in  great  beauty. 

These  solutions  in  the  horizontal  tank  should  not  be 
more  than  an  eighth  of  an  inch  deep. 


HEATING    BY   THE    CURRENT. 

Make  a  small  coil  of  platinum  wire,   and  thrust  the 
ends  of  the  wire  through  the  rubber  of  the   tank,  as 


Fig.  112. 

shown  in  the  engraving,  Fig.  112.     Fill  the  tank  with 
water,  and  having  projected  the  whole,  send  the  current 


156  THE  ART  OF  PROJECTING. 

through  the  wire.  If  the  current  is  sufficiently  great 
the  wire  coil  will  be  heated  at  once,  and  a  convection 
current  will  at  once  show  itself  in  the  water,  the  heated 
water  next  to  the  wire  rising  rapidly  to  the  top.  The 
effect  will  be  still  more  marked  if  a  drop  or  two  of 
some  one  of  the  aniline  dyes  be  let  fall  from  the  surface 
over  the  wire.  Its  greater  density  will  carry  it  at  once 
to  the  bottom ;  but  when  the  current  is  sent  through 
the  wire,  the  movements  in  the  water  will  be  rendered 
very  plain.  The  bichloride  of  tin  or  the  sulphate  of 
zinc  will  also  answer  the  same  purpose. 


CHEMISTRY. 

Most  of  the  chemical  reactions  that  are  usually  ex- 
hibited before  classes  in  the  recitation  or  lecture-room 
can  be  shown  in  a  much  more  satisfactory  way  by 
means  of  the  apparatus  for  projection  than  in  the  ordi- 
nary way.  The  method  is  moreover  both  cheaper  and 
easier;  cheaper,  because  each  experiment  requires  but 
a  few  drops  of  the  substance  in  a  test-tube  or  the 
tank,  instead  of  the  large  quantity  necessary  for  many 
to  see  at  once,  and  easier,  because  the  preparation 
needed  for  experiments  upon  an  extended  scale  is 
always  tedious  and  tiresome.  One  who  uses  the  tank 
(Fig.  20)  for  the  first  time  for  projection,  say  of  sil- 
ver, in  a  solution  as  dilute  as  two  or  three  drops  of  the 
nitrate  to  the  tank  full  of  water,  will  be  surprised  at 
the  prodigious  precipitation  brought  about  by  the  addi- 
tion of  a  single  drop  of  hydrochloric  acid  introduced 
upon  the  end  of  a  glass  rod.  Great  heavy  clouds  roll 
and  tumble  about  upon  the  screen,  looking  as  though 
they  might  weigh  tons. 


CHEMISTRY.  157 

ACIDS    AND    ALKALIES. 

Nearly  fill  the  tank  with  water  and  add  a  few  drops 
of  blue  litmus  solution;  then  dip  a  glass  rod  into  a 
weak  acid  solution  of  any  convenient  kind  and  gently 
stir  the  litmus  solution  with  it :  it  will  turn  red  in  the 
neighborhood  of  the  rod.  After  washing  the  rod,  dip 
it  into  an  alkaline  solution  of  ammonia  or  potash,  and 
again  stir  the  solution  in  the  tank.  Blue  clouds  will 
form  in  the  red  sky  upon  the  screen  until  the  whole  is 
again  a  beautiful  blue. 

In  place  of  litmus  solution  use  a  solution  made  by 
boiling  purple  cabbage.  Acid  turns  this  red,  and  an 
alkali  turns  it  green.  Such  changes  may  be  effected  a 
number  of  times  in  succession  in  the  same  solution. 

Nearly  fill  the  tank  with  sulphate  of  soda,  in  which 
is  put  either  litmus  or  cabbage  solution  to  color  it ;  the 
latter  is  the  best.  After  projecting  it  as  a  blue  solu- 
tion dip  the  terminals  of  a  battery  of  three  or  four 
cells  into  it.  Decomposition  will  begin  and  the  acid 
and  alkaline  reactions  will  be  observed  about  the  poles. 

REACTIONS   AND   PRECIPITATION. 

Fill  the  clean  tank  nearly  full  of  pure  water  and  add 
a  drop  or  two  of  a  solution  of  nitrate  of  silver  and  stir 
it  well.  Then  dip  the  glass  rod  into  very  dilute  hydro- 
chloric acid.  Very  dense  clouds  of  chloride  of  silver 
will  form  and  fall  to  the  bottom  of  the  tank.  Add  a 
few  drops  of  strong  ammonia  water,  and  the  cloudy 
solution  will  again  become  clear. 

A  small  piece  of  carbonate  of  lime  or  of  soda  placed 
in  the  tank  containing  a  very  dilute  solution  of  hydro- 
chloric acid  gives  up  its  carbonic  acid  in  apparently 
large  quantities 


158  THE  ART  OF  PROJECTING. 

To  water  made  slightly  acid,  add  enough  litmus 
solution  to  turn  it  red  and  project  it ;  then  drop  a  lump 
of  carbonate  of  ammonia  into  it.  It  will  dissolve 
rapidly  with  effervescence,  and  the  solution  will  be 
made  blue  about  the  crystal,  and  if  there  is  enough  of 
it  the  whole  solution  will  ultimately  become  blue. 

The  gradual  solution  of  substances  in  water  may  be 
nicely  shown  by  filling  the  tank  with  pure  water  and 
dropping  a  crystal  of  alum  or  sulphate  of  zinc  or  sul- 
phate of  copper  into  it.  Where  the  substance  is  dis- 
solved the  solution  will  be  denser,  and  its  refractive 
powers  changed,  which  will  be  manifest  by  gently  stir- 
ring it  with  a  glass  rod. 

A  dilute  solution  of  copper  sulphate  may  be  placed 
in  the  tank.  With  a  pipette,  force  into  the  solution 
some  ammonia  water :  A  dense  precipitate  will  at  first 
be  formed,  which  will  afterwards  be  dissolved  if  am- 
monia enough  has  been  added,  leaving  the  solution  a 
beautiful  blue  color.  A  few  drops  of  sulphuric  acid 
will  reproduce  the  precipitate  and  destroy  the  color ; 
and  when  the  solution  again  becomes  clear,  a  few  drops 
of  ferrocyanide  of  potassium  added  will  produce  a 
brownish-red  bulky  precipitate,  which  will  present  a 
fine  appearance  upon  the  screen. 

In  like  manner  all  of  the  characteristic  reactions  of 
inorganic  chemistry  may  be  projected,  and  often  with 
much  less  expenditure  of  materials  than  would  be  used 
if  large  vessels  were  employed  to  demonstrate  the 
same  things.  One  who  has  projected  a  number  of 
these  phenomena  will  find  no  diflaculty  in  projecting 
any  reaction  that  may  be  observed  in  a  test-tube. 

Pictures  of  chemical  apparatus,  of  processes,  etc., 
will  be  very  convenient  for  projection  when  instruction 
is  given  in  chemistry. 


ELECTRIC  LIGHTS. 


'39 


ELECTRIC    LIGHTS   FOR 
PROJECTION. 


Since  this  book  was  first  published  Electric  lighting 
has  become  a  great  industry,  and  most  remarkable 
advances  have  been  made  in  the  economy  of  pro- 
duction of  electricity,  and  in  the  devices  for  its  utiliza- 
tion. Compare  the  statement 
made  on  pages  9  and  10  with 
what  any  one  may  see  in  any 
city  and  in  hundreds  of  towns 
here  and  in  Europe.  Arc  lights 
of  great  steadiness  are  made  by 
many  makers,  and  the  carbons 
^adapted  to  them  are  plentiful 
and  to  be  had  for  a  few  cents 
apiece.  Consequently  one  may 
now  have  an  arc  light  for 
projection  experiments  in  al- 
most every  place.  A  regulator 
is  not  specially  needed,  for  the 
carbons  burn  but  slowly,  about 
an  inch  an  hour,  and  hand 
regulation  does  not  much  inter- 
fere even  with  extended  lec- 
tures,   while    the    brilliancy  of 

the      pictures      surpasses     many  for  Projection. 

times  the  best  possible  with  the  oxyhydrogen  light, 
but  an  automatic  regulator  is  a  great  convenience. 
The   ordinary  electric  lamps  are  so  made  as  to  feed 


Ha-wkkkksk's  Electric   Lamps 


i6o 


THE  ART  OF  PROJECTING. 


by  the  movement  of  the  upper  carbon  alone,  and  this 
will  not  answer  at  all  for  lantern  work.  Both  carbons 
must  move.     The  annexed  cuts  represent  electric  arc 

lamps  designed  to  do  this. 
Marcy  has  also  an  electric 
lamp  in  which  the  upper  car- 
bon is  inclined  so  as  to 
present  the  concave  surface 
of  the  glowing  carbon  to  the 
condense*,  which  device  ap- 
pears to  work  well.  A  good 
electric  arc  gives  light  equal 
to  about  a  thousand  stand- 
ard candles,  while  very 
ordinary  ones  give  five  or 
seven  hundred.  For  most 
teachers'  uses,  however,  the 
steady  projection  of  trans- 
parencies is  seldom  needed, 
but  an  electric  light  for 
common  purposes  that  may 
be  had  by  simply  turning  a 
switch  is  highly  desirable. 
There  is  said  to  be  an  ad- 
vantage to  be  derived  from 
combining  the  arc  light  with 
the  incandescence  of  lime, 


Queen's  Electric  Lamp. 


the  latter  giving  a  degree  of  steadiness  and  a  brighter 
light  with  a  given  current  than  would  be  had  without 
it.  The  block  of  lime  has  a  hole  through  it  large  enough 
to  allow  the  carbons  to  move  loosely  in  it.  Near  the 
middle  of  the  block,  on  one  side,  a  hole  is  cut  through 
to  meet  the  other,  and  it  is  opposite  to  this  hole  that 
the  carbons  are  to  touch  and  the  arc  be  formed,  shin- 


ELECTRIC  LIGHTS. 


i6i 


ing  through  the  front  hole  as  through  a  window.  The 
lime  soon  gets  white  hot,  and  adds  its  luminosity  to 
that  of  the  carbons.  If  there  be  a  degree  of' un- 
steadiness in  the  arc  itself  the  lime  does  not  so  quickly 


Electric  Lamp  in  Lantern. 

cool,  and  the  field  is  kept  bright  until  the  current  is 
fully  established  again. 

TO    PROJECT   THE    ARC    LIGHT. 

It  is  only  necessary  to  place  an  ordinary  lens  three 
or  four  inches  in  diameter  and  a  foot  focus  —  that 
is,  the  ordinary  projecting  lens  described  on  page  25  — 


l62  THE   ART  OF  PROJECTING. 

near  the  light  and  between  it  and  the  screen,  and  focus 
it  in  the  way  indicated  on  page  25.  The  incandescent 
carbons  will  show  beautifully  and  between  them  the 
moving  bluish  arc.  For  this  experiment  the  white 
wall  of  a  room,  in  other  directions  than  where  the 
screen  may  be,  will  be  found  to  be  a  good  surface  to 
receive  the  image  upon.  The  source  of  light  is  so 
bright  that  the  most  distant  place  in  the  room  will  show 
it  plainly  enough,  and  the  more  distant  the  image  is 
the  larger  it  will  be. 

The  incandescent  filament  may  be  projected  in  a 
similar  manner,  and  will  show  as  an  inverted,  glowing 
loop. 

The  common  incandescent  electric  light  does  not  give 
light  enough  to  enable  one  to  use  it  in  a  lantern.  Most 
of  them  give  a  light  of  but  fifteen  or  twenty  candles. 
Those  that  give  more  have  a  filament  so  long  that 
its  use  in  a  lantern  is  quite  impracticable,  not  alone 
on  account  of  size  of  the  bulb,  but  because  the  source 
of  the  light  is  from  so  large  an  area  that  definition  is 
impossible.  Lamps  may  be  made,  though,  having  the 
luminous  filament  reduced  to  a  small  area,  like  a  coil, 
thus  : 


When  this  can  be  done  so  that  the  luminous  area 
does  not  much  exceed  an  inch  in  diameter,  a  very 
good  source  of  light  is  provided.  But  if  common  fila- 
ments are  made  into  this  shape  they  must  be  supplied 
with  a  much  larger  current  than  they  are  usually  sup- 
plied with,  and  they  will  not,  therefore,  last  so  long. 
A  filament  about  six  inches  long  is  intended  to  give 
about  sixteen   candles'  light   or  nearly  three    candles 


ELECTRIC  LIGHTS. 


163 


to  the  inch  of  filament.  By  increasing  the  current  the 
light  increases  very  rapidly,  so  that  by  doubling  it  the 
light  may  be  made  equal 
to  100  candles  or  more,  that 
is,  sixteen  candles  or  more, 
to  the  inch  of  filament. 
When  filaments  are  made 
tubular,  like  Bernstein's, 
they  may  be  made  much 
shorter.  Such  an  one,  hav- 
ing a  length  of  three  inches, 
bent  into  a  U  form  may  give 
a  light  equal  to  300  candles, 

—  100  candles  to  the  inch, 

—  and  this  answers  for 
projections  where  the  de- 
tails of  the  picture  are  not 
too  minute.  It  will  not 
answer  well  for  micro- 
scopic projections,  but  for 
common  transparencies 
works  well  enough. 

When  such  a  lamp  is 
placed  in  the  lantern  and 
moved  towards  the  con- 
denser, the  light  upon  the 
screen  will  increase  to  a 
maximum,  when  the  en- 
larged image  of  the  fila- 
ment will  appear,  and  the 
disc  will  not  be  uniformly 

lighted.  The  lamp  should  therefore  be  drawn  back 
a  little  to  secure  a  uniform  field.  This  will  be  at 
the  sacrifice  of  some  of  the  light,  but  the  brightness 


Bernstein's  Electric  Lamp. 


1 64  THE  ART  OF  PROJECTING. 

is  then  equalled  by  only  a  very  good  oxyhydrogen 
lime  light. 

Electric-light  plants  are  now  to  be  found  in  most 
cities  and  large  towns,  and  in  a  short  time  will  be  found 
in  every  town  and  village.  It  will  therefore  be  possible 
for  every  one  to  use  electricity  for  his  source  of  light  for 
projecting.  Different  electric-light  companies  use 
currents  of  different  strengths  for  their  service,  and 
at  present  there  is  nothing  like  uniformity  among  them. 
As  an  electric  lamp  needs  to  be  adapted  to  the  current 
it  is  supplied  with  in  order  that  it  should  give  its  proper 
amount  of  light,  the  maker  of  it  must  know  what 
current  the  lamp  is  to  be  supplied  with.  The  lamp 
filament  is  a  conductor  of   electricity,  and  as  such  is 

subject  to  Ohm's  Law,  namely,  -  =  C,  when  E  is  the 

difference  in  electric  potentials  between  the  terminals 
of  the  lamp,  R  the  resistance  of  the  filament,  and  C  the 
strength  of  the  flowing  current.  The  electric  energy  in 
the  lamp  equals  EC,  and  is  reckoned  in  units  called 
watts.  Ordinary  incandescent  lamps  require  three  or 
four  watts  per  candle,  but  by  increasing  the  current 
through  them  the  luminosity  increases  at  a  higher  rate, 
and  may  easily  be  made  a  watt  per  candle.  This 
shortens  the  life  of  the  lamp,  but  for  lantern  purposes 
that  is  of  but  little  consequence.  That  is  to  say,  a  lamp 
run  at  the  rate  of  one  volt  per  candle  will  last  fifty 
or  one  hundred  hours.  It  will  always  be  prudent  to 
have  two  or  three  lamps  at  hand.  In  case  the  one  in 
use  should  suddenly  collapse  another  may  instantly  be 
substituted  and  with  no  awkward  delay.  I  shall 
assume  here  that  every  lamp  used  for  lantern  projec- 
tions will  be  so  adapted  to  the  current  provided  for  it 
as  to  yield  a  candle    for  a  watt,  thus,  EC  ^  watts  = 


ELECTRIC  LIGHTS.  165 

candle  power,  and  the  value  of  C  will  always 
be  known.     Let  W  equal  candle  power  required,  then 

W 

E  =  — .     The    difference    of   potentials   E   equals    the 

candle    power   divided   by  the   current   provided.     As 

R  =  -,  the  resistance  of  the  filament  may  be  computed, 

remembering  that  the  resistance  of  the  filament  while 
hot  is  but  about  one-half  of  what  it  is  when  it  is  cold. 
Suppose  the  electric-light  company  in  the  neighborhood 
provides  a  ten-ampere  current,  what  must  be  the  resist- 
ance of  the  lamp  in  order  to  give  say  300  candles  ? 
E  =  ^^^^  30  volts  must  be  the  difference  of  potentials 
and  R=3ft  =  2  ohms  must  be  the  resistance  of  the 
filament  while  hot.  It  will  be  five  or  six  ohms  when 
cold.  In  this  way  one  may  adapt  his  lamp  to  currents 
of  other  degrees  of  strength.  In  ordering  a  lamp,  how- 
ever, it  will  always  be  best  to  specify  the  current  strength 
at  command  and  the  candle  power  wanted. 

SPECTRA    OF   THE    ELEMENTS. 

By  making  the  terminals  of  an  induction  coil  of 
different  metals,  sparks  from  them  will  give  their  char- 
acteristic spectra.  Arrange  then  a  lens  so  as  to  pro- 
ject the  spark  upon  the  screen,  as  if  the  spark  were  a 
common  object.  Then  near  the  focus  of  the  lens  place 
the  prism  so  as  to  deflect  the  rays.  The  dispersion  will 
at  once  be  apparent  as  there  will  be  as  many  images  of 
the  spark  as  there  are  visible  rays.  The  zigzag  form 
of  the  spark  will  be  duplicated  in  each  bright  line.  If 
the  terminals  of  a  condenser  like  a  leyden  jar  be  con- 
nected to  the  terminals  of  the  induction  coil  as  is 
usual  for  brightening  the  spark,  the  latter  will  be 
shortened  very  much,  and  the  spectrum  made  brighter, 


1 66  THE  ART  OF  PROJECTING. 

appearing  more  like  colored  spots  upon  the  screen, 
than  characteristic  lines.  Very  small  fragments  of 
metals  or. other  conducting  substances  may  be  used  in 
this  way.  It  is  not,  however,  to  be  understood  that 
such  spectra  can  be  made  large  like  those  produced 
by  the  oxyhydrogen  or  electric  light.  They  may,  how- 
ever, be  shown  as  spectra  a  foot  long  and  the  lines  two 
or  three  inches  long,  and  thus  be  useful  in  places 
where  the  more  pretentious  ways  of  projecting  spectra 
are  not  to  be  had. 

If  an  induction  coil  capable  of  giving  a  spark  two  or 
three  inches  long  is  not  to  be  had,  a  common  electrical 
machine  will  answer ;  for  the  elements  to  be  employed 
may  be  fastened  into  retort  stands,  and  separated  an 
inch  or  two  as  if  simply  to  pass  sparks  from  one  to  the 
other,  these  connected  by  wires  to  the  Holtz  or  other 
similar  electrical  machine.  The  sparks  may  be  pro- 
jected as  in  the  case  with  induction  coils. 

TO    PROJECT   AN    ELECTRIC    SPARK, 

Suppose,  from  a  Holtz  machine.  So  place  the 
machine  that  the  spark  between  the  terminals  shall  be 
parallel  to  the  screen  to  receive  the  image.  Take  a 
lens  with  a  foot  focus  and  with  as  large  a  diameter  as 
possible,  say  four  or  five  inches,  and  mounted  in  a 
broad  frame  (Fig.  15).  If  this  be  placed  at  the  proper 
distance  and  height  from  the  terminals  of  the  electric 
machine,  a  spark  will  be  projected  on  the  screen  much 
magnified,  all  its  zigzag  lines  amplified.  There  is  no 
difficulty  in  making  an  ordinary  three  or  four  inch 
spark  appear  to  be  six  or  eight  feet  long  if  the  screen 
be  fifteen  or  twenty  feet  away.  It  will  be  necessary  to 
have  the  room  quite  dark,  and  also,  to  have  the  screen 
shielded  from  the  light  of  the  spark,  but  the  frame  of 


ELECTRIC  LIGHTS.  1 67 

the  lens  may  be  sufficient.     If  it  is  not,  a   screen  of 
paste-board  or  something  similar  may  be  extemporized. 

FLOATING     MAGNETS    (mAYER's    EXPERIMENT). 

Magnetize  six  or  eight  cambric  needles  so  that  their 
points  will  all  have  similar  poles.  Thrust  these  needles 
through  small  vial  corks  so  that  when  placed  in  a  dish 
of  water  they  will  float  with  similar  poles  up.  Thus 
placed  they  will  repel  each  other  and  move  as  far 
apart  as  possible.  Bring  a  small  bar  magnet  over 
them  so  that  the  adjacent  pole  will  be  the  opposite  of 
that  of  the  upper  ends  of  the  needles.  The  needles 
will  be  attracted  by  it  and  approach  it,  but  repelling 
each  other  they  will  arrange  themselves  in  certain 
symmetrical  order,  which  will  depend  upon  the  number 
of  the  floating  magnets.  If  there  be  but  three  of  them 
they  will  assume  a  triangular  form.  If  there  be  four,  a 
square.  If  five  or  six,  there  will  be  two  or  three  posi- 
tions of  stability.  To  project  these  motions  and  forms 
it  will  be  necessary  to  have  a  glass  tank  similar  to  the 
one  described  on  page  47  for  cohesion  experiments 
with  the  vertical  attachment  to  the  lantern.  The  tank 
must,  of  course,  be  deep  enough  to  allow  the  needles  to 
float  freely  about.  The  needles  may  be  short  —  an 
inch  long.  If  the  corks  be  half  an  inch  in  diameter 
and  quarter  of  an  inch  thick,  they  will  float  in  three- 
quarters  of  an  inch  of  water  without  danger  of  over- 
turning. The  controlling  magnet  need  not  be  a  heavy 
one.  One  made  of  a  stout  knitting-needle  will  answer, 
and  it  will  be  best  for  projecting  purposes  if  the  con- 
trolling pole  be  bent  at  a  right  angle  for  two  inches  of 
its  length.  This  will  allow  proper  movement  of  it  in 
the  field  without  obscuring  the  field  by  large  shadows. 
(See  Mayer's  experiments,  Amer.  Jour,  of  Science 
1878). 


l68  THE  ART  OF  PROJECTING. 

LESSENING    CHROMATIC    ABERRATION. 

When  a  double  convex  lens  is  used  as  an  objective 
as  described  on  page  25,  the  parts  of  the  picture  upon 
the  screen  near  the  margin  of  the  disc  will  be  seen  to 
have  many  of  the  lines  brightly  colored  with  spectrum 
tints.  At  a  distance  of  fifteen  or  twenty  feet  from  the 
screen  these  spectral  colors  do  not  give  much  trouble, 
but  nearer  they  are  oftentimes  objectionable.  By  using 
a  compound  objective,  such  as  is  made  for  lanterns,  and 
especially  for  photographic  work,  all  this  may  be 
avoided  ;  but  such  compound  objectives  cost  considera- 
ble. If  one  cannot  afford  such  a  lens,  he  can  use  two 
similar  lenses  having  the  same  or  nearly  the  same  focal 
length.  Use  one  of  these  for  a  condenser,  placing  the 
picture  close  to  it,  and  permit  the  converging  rays  to  pass 
through  the  middle  of  the  lens  used  as  an  objective. 
The  trouble  willlargely.be  prevented,  especially  if  the 
objective  be  covered,  except  a  round  or  square  hole  at 
its  middle,  so  that  no  light  will  pass  to  the  screen  except 
what  goes  through  the  orifice  and  the  middle  of  the 
lens.  A  plano-convex  lens  two  or  three  inches  in  di- 
ameter, and  with  a  focus  of  eight  or  ten  inches,  so  used, 
will  do  as  well  as  a  combination  achromatic  costing  ten 
or  twenty  times  as  much.  P'or  microscopic  projections, 
small  objectives,  such  as  are  used  for  taking  multiple 
tin-types,  answer  nearly  as  well  as  the  more  costly  ones. 
They  may  be  had  with  ratchet  movements  for  about 
five  dollars,  and  without  the  ratchet  for  much  less. 
Their  focal  length  is  about  an  inch. 

BUBBLE   COHESION. 

A  group  of  soap  bubbles  in  contact  with  each  other 
cohere    together,    and    their    surface    tension    always 


ELECTRIC  LIGHTS.  169 

organizes  them  into  a  symmetrical  arrangement,  with 
plane  sides  at  their  junctions,  instead  of  curved  sides, 
as  single  ones  have. 

Pour  a  few  drops  of  soap  solution  upon  a  piece  of 
window-glass,  and  with  the  finger  spread  it  over  a  sur- 
face four  or  five  inches  square.  Then  with  a  common 
blow-pipe  or  glass  dropping-tube  blow  bubbles  upon  the 
glass,  starting  them  at  any  point.  They  had  better  not 
be  blown  more  than  an  inch  or  two  in  diameter.  If  a 
second  bubble  be  started  at  a  point  an  inch  or  two  from 
the  first  one,  the  two  will  rush  together ;  a  third  one 
will  join  to  the  two  so  that  the  interior  angles  at  their 
junction  will  equal  120°.  As  others  are  added  all  will 
change  their  surfaces  of  adhesion  and  their  relative 
positions. 

By  placing  the  glass  upon  the  vertical  projector  (pp. 
42  and  43),  the  growth,  motions,  and  symmetrical  ar- 
rangement may  be  seen  and  studied  by  a  hallful  at  once. 

VIBRATION    OF    FILMS. 

If  the  end  of  a  tube  like  a  glass  lamp-chimney  be 
dipped  into  a  soap  solution,  a  film  will  remain  over  the 
end  when  it  is  taken  out  of  it.  If  now  a  beam  of  par- 
allel rays  of  light  be  directed  upon  this  film,  some  of  the 
light  will  be  reflected  from  it.  Place  a  lens  four 
or  five  inches  in  diameter  and  ten  or  twelve  inches 
focus  so  as  to  project  this  reflected  light  upon  the  screen 
or  white  wall.  An  enlarged  image  of  the  film  will  be 
seen  upon  which  a  series  of  spectral  colors  will  appear. 
If  a  sound  be  made  by  the  voice  at  the  open  end  of 
the  tube,  the  film  will  be  thrown  into  vibrations  similar 
in  form  to  the  air  waves  that  produce  them.  These 
vibratory  movements  will  show  upon  the  screen  as 
a  curious  network  which  will  change  for  each  different 


170  THE  ART  OF  PROJECTING. 

kind  of  sound.  The  changes  in  the  patterns,  combined 
with  the  many  colors  of  the  fihn,  make  interesting 
studies  in  acoustics.  It  is  important  that  the  tube  upon 
which  the  fihns  are  made  should  be  fixed  while  the 
projections  are  looked  at,  for  otherwise  the  coruscations 
cannot  be  seen.  It  will  be  sufficient,  however,  to 
fasten  it  in  a  retort  holder.  As  the  light  for  the  pro- 
jection is  reflected  from  the  surface,  it  will  be  best  not 
to  have  the  reflected  beam  more  than  about  ninety 
degrees  from  the  incident  beam,  otherwise  the  pro- 
jection will  appear  too  oval,  it  will  also  be  less  distinct. 

LANDSCAPE    PROJECTION. 

In  the  experiments  to  illustrate  rectilinear  move- 
ment of  light,  on  page  81,  it  is  remarked  that  the 
appearance  of  the  landscape  as  shown  on  the  walls  of 
the  room  is  much  brighter  when  snow  is  upon  the 
ground.  The  definition  is  made  much  better  by  making 
the  orifice  small,  but  then  the  light  is  so  much  scattered 
that  the  images  are  not  very  distinct.  By  employing  a 
lens  with  as  long  a  focus  as  possible,  and  placed  at  the 
orifice,  a  beautiful  image  of  the  external  landscape  will 
appear  upon  a  screen  at  a  proper  distance,  A  lens 
with  almost  any  focal  length  will  show  good  definition, 
but  the  projection  will  be  too  small  for  any  considerable 
number  to  see  at  once.  A  lens  with  six  or  eight  feet 
length  of  focus  will  show  a  picture  five  or  six  feet 
square  with  all  the  details  of  the  landscape  easily 
discernible  in  a  darkened  room. 

SODIUM    LINE    IN    SOLAR    SPECTRUM. 

Having  arranged  the  apparatus  as  shown  in  Fig. 
89,  for  showing  the  more  prominent  Fraunhofer  lines, 


ELECTRIC  LIGHTS.  171 

ignite  a  piece  of  sodium  as  large  as  half  a  pea,  and 
hold  it  while  burning  so  that  the  light  from  the  sun  must 
pass  through  the  flaming  sodium  that  is  immediately 
in  front  of  the  slit.  The  yellow  of  the  sunlight  will  be 
stopped,  and  a  large  and  densely  black  line  will  be 
seen  in  the  place  of  the  yellow  in  the  spectrum.  A  very 
good  way  to  ignite  the  sodium  is  to  provide  a  soft  pine 
stick,  six  or  eight  inches  long  and  half  an  inch  or  more 
thick.  Close  to  one  end  cut  out  a  hole  large  enough  to 
hold  the  bit  of  sodium  to  be  used,  and  crowd  this  into 
it.  The  end  of  the  stick  can  be  lighted  in  a  gas  or 
alcohol  flame,  and  then  hastily  moved  to  the  position 
where  it  is  needed.  The  inflamed  wood  will  set  fire 
to  the  sodium  in  a  few  seconds,  when  it  will  burn  with 
a  great  flame  and  dense  fumes,  yet  without  endangering 
the  hand.  The  yellow  flame  and  the  light  from  it 
will  not  seriously  impair  the  appearance  of  the  spectrum 
upon  the  screen. 


RELATION     BETWEEN     SIZE    OF    OBJECT,    SIZE    OF     IMAGE, 
AND    FOCAL    LENGTH    OF    OBJECTIVE. 

It  is  often  convenient  to  know  how  large  a  given 
picture  will  be  upon  the  screen  when  projected,  what 
kind  of  an  objective  to  use  to  obtain  a  picture  of  a 
definite  size,  and  so  on.  The  following  rule  will  enable 
one  to  know  and  provide  such  conditions. 

Let  A  represent  the  focal  length  of  the  objective ;  let 
B  represent  the  distance  from  the  objective  to  the 
screen  ;  let  C  represent  the  diameter  of  the  space  to 
be  projected;  let  D  represent  the  diameter  of  the 
lighted  space  upon  the  screen.  Then,  as  A  :  B  : :  C  :  D. 
Three  of  these  will  nearly  always  be  known.  Suppose 
the    transparency    to   be    projected    be   3    inches    in 


172  THE  ART  OF  PROJECTING. 

diameter,  the  lens  to  be  one  foot  focus,  the  distance  to 
the  wall  25  feet.  How  large  will  the  screen  need  to  be 
to  receive  the  image  ?  As  i  :  25  : :  3  :  x  =  75  inches  = 
6^  feet.  Suppose  the  object  to  be  one  inch  long. 
What  must  be  the  focal  length  of  a  lens  to  project  the 
image  4  feet  long  when  the  screen  is  20  feet  away  ?  As 
48  :  I  : :  240  :  x  =  5  inches.  In  this  way  one  may  pro- 
vide distance,  screen,  and  lenses  to  suit  his  con- 
venience near  enough  for  all  practical  purposes. 

VORTEX    RINGS    AND    THEIR    PHENOMENA. 

The  phenomena  presented  by  vortex  rings  are  so 
interesting  —  some  of  them  so  surprising  —  and  yet  are 
so  easily  produced  as  to  warrant  giving  some  space  to 
an  account  of  their  production.  Aside  from  this  the 
physical  importance  of  their  study  is  very  great,  seeing 
that  Sir  Wm.  Thomson  and  others  have  seriously 
proposed  to  account  for  the  properties  of  atoms  of  mat- 
ter by  supposing  the  latter  to  be  vortex  rings  of  ether  in 
the  ether.  The  ring  of  steam  often  seen  puffed  from 
a  locomotive,  and  rising  in  the  air  sometimes  a  hundred 
feet  or  more,  is  called  a  vortex  ring  or  sometimes 
a  smoke  ring.  Such  rings  are  formed  whenever  a  gas 
or  a  liquid  is  suddenly  pushed  through  an  orifice.  If 
they  be  formed  solely  of  air  they  cannot  be  seen,  on 
account  of  their  transparency,  but  they  may  be  made 
manifest  in  other  ways. 

Over  the  mouth  of  a  glass  or  a  tin  funnel  three  or 
four  inches  in  diameter,  tie  a  piece  of  stout  paper. 
Snap  with  the  finger  upon  this  stretched  paper  and  a 
ring  will  be  projected  from  the  stem.  If  the  latter 
be  directed  towards  the  face  it  will  be  easily  felt,  and 
if  it  be  directed  towards  a  candle  flame  it  may  blow  it 


ELECTRIC  LIGHTS. 


173 


out  at  a  distance  of  three  or  four  feet.  With  a  larger 
funnel  and  stronger  blow  it  may  extinguish  the  flame  at 
a  greater  distance.  To  make  them  visible  it  is  neces- 
sary to  mix  the  fumes  of  something,  smoke  for  instance, 
with  the  air  of  which  they  are  formed.  Very  good 
ones  may  be  formed  by'  the  mouth.  Let  the  mouth 
be  filled  with  smoke  and  the  lips  be  pursed  as  if  to 
produce  the  vowel  0,  then  tap  the  cheek  with  the  end  of 
the  finger,  and  smoke  rings  an  inch  or  two  in 
diameter  will  be  formed.  Some  smokers  are  able 
to  project  very  large  and  dense  ones  from  their 
mouth  by  a  sudden  forward  thrust  of  the  back  of 
the  mouth,  a  movement  which  has  to  be  acquired  by 
practice. 

For  the  production  of  vortex  rings  for  the  study  of 
their  behavior,  it  will  be  necessary  to  have  made  a 
box  which  may  be  kept  filled  with  the  visible  vapor. 
One  of  the  following  shape  and  dimensions  will  be  found 
to  answer  well.  A  box  of  wood 
about  a  foot  cube,  having  a 
round  hole  about  four  inches  in 
diameter  cut  in  the  middle  of 
one  side.  A  swinging  hinged 
back,  framed  square,  over  which 
may  be  stretched  tightly  some 
stout  cotton  cloth,  will  close 
the  box  tightly  enough  when  it 
is  (iown.  The  cut  represents  the  back  of  the  box 
lifted  up.  It  will  be  convenient  to  have  two  strips 
in  front  grooved  so  as  to  permit  a  slide  to  be  inserted. 
Several  slides  may  be  made  for  this  position,  each  one 
having  its  orifices  through  which  the  smoke  may  be  pro- 
jected. The  following  are  suggested  as  being  useful. 
One  with  a  round  hole  three  inches  in  diameter.     One 


174  THE  ART  OF  PROJECTING. 

with  an  oval  hole  three  inches  in  its  longer  and  two 
inches  in  its  shorter  diameter.  One  with  two  holes  one 
inch  in  diameter  and  an  inch  and  a  half 


O  O  apart,  the  two  holes  horizontal.     Oue 

with  two  holes  like  the  last  except  that 
one  is  to  be  over  the  other.  One  with  three  holes  each 
an  inch  in  diameter,  their  centres  two  and  a  half  inches 
apart.  One  with  a  hole  two  inches  square.  Two 
saucers  or  other  crockery  vessels  presenting  as  large 
a  fluid  surface  when  filled  as  convenient,  may  be  filled, 
one  with  the  strongest  ammonia  water,  the  other  with  the 
strongest  hydrochloric  acid,  and  placed  in  the  box 
and  the  back  closed.  The  box  will  at  once  be  filled  with 
the  dense  white  vapor  of  ammonium  chloride.  If  the 
solutions  be  heated  before  being  placed  in  the  box  the 
fumes  will  be  denser  still,  and  therefore  better  for  this 
purpose. 

EXPERIMENTS. 

1.  Strike  the  cloth  back  of  the  box  with  the  hand 
suddenly.  A  white  ring  five  or  six  inches  in  diameter 
will  be  projected  and  will  move  several  feet.  If  the 
smaller  three  inch  hole  be  in  front,  the  ring  will  be 
smaller  and  will  move  faster. 

2.  Produce  a  ring  by  swinging  the  back  of  the  box 
an  inch  or  two  and  letting  it  strike  the  box  smartly. 
The  ring  will  move  with  rapidity  fifteen  or  twenty  feet 
in  the  air,  going  in  a  straight  line  if  there  be  no 
currents  of  air  to  deflect  it  or  objects  near  to  its  path. 

3.  If  the  table  be  ten  feet  long  or  more  and  the  box 
be  at  one  end  of  it,  so  that  the  rings  may  move  over  the 
length  of  the  table,  a  swift  moving  ring  will  come 
down  to  it  and  be  broken,  as  if  the  table  attracted  it. 
To  prevent  this,  tilt  up  the  box,  being  careful    about 


ELECTRIC  LIGHTS.  17$ 

spilling  the  contents  of   the  saucers  if   they  are  very 
full. 

4.  Make  one  ring  to  follow  another  so  as  to  overtake 
it.  If  the  axes  of  the  two  coincide,  the  forward  one  will 
expand,  while  the  oncoming  one  will  contract  in  diam- 
eter, permitting  the  latter  to  go  through  the  forward 
and  larger  one,  when  each  will  assume  its  original 
dimension. 

5.  Project  one  ring  after  another  so  that  they  may  col- 
lide. Each  will  be  seen  to  be  deformed  and  each  will 
vibrate,  assuming  oval  shapes  with  axes  at  right  angles 
to  each  other,  thus  indicating  that  the  rings  are  elastic. 

6.  Project  a  ring  so  that  it  will  pass  near  a  suspended 
fibre  of  thread  or  other  light  body.  The  thread 
will  appear  to  be  repelled  from  the  front  and  attracted 
by  the  back  of  the  ring. 

7.  A  ring  formed  by  the  oval  hole  will  move  forward 
like  the  round  one,  but  will  vibrate  energetically,  going 
through  the  phases  mentioned  in  experiment  5. 

8.  The  triangular  hole  will  likewise  give  a  vibrating 
ring,  as  will  one  generated  with  any  other  form  of 
orifice,  so  that  it  is  impossible  to  have  a  ring  that  will 
maintain  any  other  form  than  the  circalar  one  with  its 
phases  of  vibration. 

9.  With  the  double  aperture  slide,  two  rings  will  be 
formed  simultaneously,  but  instead  of  producing  them 
as  the  larger  ones  were,  they  can  best  be  made  by 
a  tap  with  the  finger  upon  the  cloth  back  near  to  its 
edge.  The  rings  will  be  small  but  well  formed,  and 
move  so  slowly  that  their  motions  may  be  easily 
watched. 

10.  Observe  that  when  the  two  are  produced  they 
ifivariably  collide,  they  never  move  off  parallel  with 
each  other. 


176  THE   ART  OF  PROJECTING. 

11.  After  a  collision  of  two  such  formed  rings  they 
may  separate,  but  when  they  do  they  always  move  away 
from  each  other  in  a  plane  at  right  angles  to  the  plane 
of  collision.  If  the  two  holes  of  the  slide  are  horizontal, 
the  rings  will  bound  from  each  other  in  a  vertical  plane. 
If  the  holes  be  vertical,  they  will  bound  away  from  each 
other  horizontally. 

12.  If  the  rings  do  not  rebound,  they  will  each  break 
at  the  point  of  contact  and  weld  together  into  a  single 
ring  having  twice  the  diameter,  and  move  on  in  a  right 
line  from  the  front  of  the  box,  but  vibrating  like  the  ring 
formed  by  the  oval  orifice. 

13.  By  using  the  slide  with  three  holes  the  rings 
may  rebound  from  each  other  after  collision  —  for 
they  will  always  collide  as  do  those  formed  from  the  two 
holes  —  or  they  may  all  combine  to  form  a  single 
ring,  each  breaking  apart  at  the  point  of  contact  with  the 
others. 

14.  Observe  that  a  ring  always  moves  plane  on  — 
that  is,  never  sideways  or  in  other  directions  than  at 
right  angles  to  a  plane  through  itself.  (Of  course  a 
ring  may  be  drifted  about  by  currents  of  air,  but  such  is 
not  the  proper  motion  of  the  ring.) 

15.  When  a  ring  strikes  upon  a  surface  parallel  with 
its  own  plane,  its  diameter  increases  indefinitely,  while 
the  cross  section  of  the  ring  gets  thinner  and  thinner. 

16.  Rings  having  sections  of  greater  density  than 
other  parts  often  show  vibratory  motions  of  such  parts. 
Two  such  on  opposite  sides  of  a  ring  will  approach 
each  other  and  combine  midway,  heaping  up  at  that 
place,  then  each  part'  retreats  from  the  other  to  meet 
upon  the  opposite  side  of  the  ring.  A  kind  of  peripheral 
vibration.  If  the  motions  be  not  very  energetic,  the 
denser  parts  may  not  separate  more  than  180  degrees. 


ELECTRIC  LIGHTS.  1 77 

17.  A  denser  part  of  a  ring  may  sometimes  be  seen 
to  be  travelling  round  the  ring  without  apparent  rotation 
of  the  ring  itself,  but  this  phenomenon  I  have  not  been 
able  to  reproduce  at  will. 

18.  Sometimes  a  spiral  movement  may  be  seen  to  be 
taking  place  —  in  and  out'as  well  as  round  the  ring. 

*  19.  If  one  is  provided  with  two  boxes  for  forming 
these  rings,  they  may  be  used  in  conjunction.  Rings 
may  be  made  to  move  towards  and  away  from  each 
other  at  any  angle  and  at  different  velocities.  Rings  of 
different  sizes  moving  towards  each  other  in  the  same 
line  present  a  singular  phenomenon.  The  smaller 
one  will  go  through  the  larger,  and  the  one  with  the  less 
velocity  will  be  brought  to  a  standstill  in  the  air, 
while  the  other  one  goes  on  with  lessened  velocity. 
After  the  moving  one  has  advanced  a  foot  or  two,  the 
arrested  one  will  again  start  up  as  if  it  had  been  pushed 
in  the  direction  it  originally  had.  Showing  in  a  curious 
way  that  the  forward  movement  of  the  vortex  ring  is 
necessitated  by  the  motion  that  constitutes  the  ring 
itself. 

The  liquids  in  the  saucers  may  get  too  dilute  to 
serve  for  experiment  for  all  the  above  indicated  ones. 
The  ammonia  water  may  get  a  crust  of  chloride  formed 
on  its  surface  which  will  need  removal.  If  the  liquids 
be  freshly  heated  they  will  again  serve  for  experiments 
for  a  few  minutes.  In  order  that  a  roomful  may  see 
the  rings  to  good  advantage,  it  will  be  better  to  have  a 
dark  background,  and  have  the  rings  lighted  by  a  beam 
of  light  parallel  to  the  general  direction  of  their  motions. 
If  the  fumes  are  not  quite  dense,  they  may  not  be  easily 
seen  at  the  distance  of  fifteen  or  more  feet.  With  such  a 
series  of  experiments  various  properties  of  matter  may 
be  illustrated.     For  example,  the  external  and  internal 


178  THE  ART  OF  PROJECTING. 

energy  of  gases,  free  path  motion,  heat  motion  (5), 
momentum,  attraction  and  repulsion  (6)  as  due  to 
motion,  gravitative  action  (3),  chemical  action 
(12),  elasticity  (5).  It  is  not  to  be  understood  that 
these  phenomena  are  the  same  as  those  mentioned, 
only  that  they  are  strikingly  like  them  in  some 
respects.  • 


PHAS.   S.    BOURNE,  ^°^|s^^ 

MANUFACTURER  OF 

LISSAJOUS   FORKS 

(Illustrated  on  page  oo) 

AND  OTHER  APPARATUS  .FOR  PROJECTION. 

^^^Send  for  Circular. 

Established  1870. 

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HOBOKEN,  N.  J., 

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1:66  The  art  of  projecting... 

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